Sidenote: your dmesg output does not indicate error:
[ 2.636093] fb0: switching to astdrmfb from OFfb vga
[ 2.636255] [drm] platform has no IO space, trying MMIO
[ 2.636258] [drm] Using device-tree for configuration
[ 2.636258] [drm] AST 2500 detected
[ 2.636261] [drm] Using Sil164 TMDS transmitter
[ 2.636266] [drm] dram MCLK=800 Mhz type=7 bus_width=16 size=01000000
[ 2.639371] ast 0005:02:00.0: fb0: astdrmfb frame buffer device
[ 2.737473] [drm] Initialized ast 0.1.0 20120228 for 0005:02:00.0 on minor 0
[ 2.859718] [drm] amdgpu kernel modesetting enabled.
[ 2.859755] amdgpu 0000:03:00.0: remove_conflicting_pci_framebuffers: bar 0: 0x6000000000000 -> 0x600000fffffff
[ 2.859758] amdgpu 0000:03:00.0: remove_conflicting_pci_framebuffers: bar 2: 0x6000010000000 -> 0x60000101fffff
[ 2.859760] amdgpu 0000:03:00.0: remove_conflicting_pci_framebuffers: bar 5: 0x600c000000000 -> 0x600c00007ffff
[ 2.859775] amdgpu 0000:03:00.0: enabling device (0140 -> 0142)
[ 2.859995] [drm] initializing kernel modesetting (NAVI10 0x1002:0x731F 0x1002:0x0B36 0xC0).
[ 2.860002] [drm] register mmio base: 0x00000000
[ 2.860003] [drm] register mmio size: 524288
[ 2.860003] [drm] PCI I/O BAR is not found.
[ 2.860013] [drm] PCIE atomic ops is not supported
[ 2.902060] [drm] set register base offset for ATHUB
[ 2.902062] [drm] set register base offset for CLKA
[ 2.902063] [drm] set register base offset for CLKA
[ 2.902064] [drm] set register base offset for CLKA
[ 2.902064] [drm] set register base offset for CLKA
[ 2.902065] [drm] set register base offset for CLKA
[ 2.902067] [drm] set register base offset for DF
[ 2.902068] [drm] set register base offset for DMU
[ 2.902069] [drm] set register base offset for GC
[ 2.902070] [drm] set register base offset for HDP
[ 2.902071] [drm] set register base offset for MMHUB
[ 2.902071] [drm] set register base offset for MP0
[ 2.902072] [drm] set register base offset for MP1
[ 2.902073] [drm] set register base offset for NBIF
[ 2.902074] [drm] set register base offset for NBIF
[ 2.902075] [drm] set register base offset for OSSSYS
[ 2.902076] [drm] set register base offset for SDMA0
[ 2.902077] [drm] set register base offset for SDMA1
[ 2.902078] [drm] set register base offset for SMUIO
[ 2.902079] [drm] set register base offset for THM
[ 2.902080] [drm] set register base offset for UVD
[ 2.902083] [drm] add ip block number 0 <nv_common>
[ 2.902084] [drm] add ip block number 1 <gmc_v10_0>
[ 2.902085] [drm] add ip block number 2 <navi10_ih>
[ 2.902086] [drm] add ip block number 3 <psp>
[ 2.902087] [drm] add ip block number 4 <smu>
[ 2.902088] [drm] add ip block number 5 <gfx_v10_0>
[ 2.902089] [drm] add ip block number 6 <sdma_v5_0>
[ 2.902090] [drm] add ip block number 7 <vcn_v2_0>
[ 2.996953] [drm] VCN decode is enabled in VM mode
[ 2.996954] [drm] VCN encode is enabled in VM mode
[ 2.996955] [drm] VCN jpeg decode is enabled in VM mode
[ 2.996958] [drm] GPU posting now...
[ 2.997016] [drm] vm size is 262144 GB, 4 levels, block size is 9-bit, fragment size is 9-bit
[ 2.997043] amdgpu 0000:03:00.0: VRAM: 8176M 0x0000008000000000 - 0x00000081FEFFFFFF (8176M used)
[ 2.997046] amdgpu 0000:03:00.0: GART: 512M 0x0000000000000000 - 0x000000001FFFFFFF
[ 2.997050] [drm] Detected VRAM RAM=8176M, BAR=256M
[ 2.997052] [drm] RAM width 256bits GDDR6
[ 2.997064] [drm] amdgpu: 8176M of VRAM memory ready
[ 2.997068] [drm] amdgpu: 8176M of GTT memory ready.
[ 2.997111] [drm] GART: num cpu pages 8192, num gpu pages 131072
[ 2.997200] [drm] PCIE GART of 512M enabled (table at 0x0000008000000000).
[ 2.997302] [drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
[ 2.997303] [drm] Driver supports precise vblank timestamp query.
[ 2.997469] [drm] ppt_offset_bytes: 3
[ 2.997471] [drm] ppt_size_bytes: 262912
[ 2.999386] [drm] use_doorbell being set to: [true]
[ 2.999451] [drm] use_doorbell being set to: [true]
[ 2.999577] [drm] Found VCN firmware Version ENC: 1.4 DEC: 3 VEP: 0 Revision: 0
[ 2.999584] [drm] PSP loading VCN firmware
[ 3.437470] [drm] reserve 0x7200000 from 0x8000400000 for PSP TMR
[ 3.961257] amdgpu: [powerplay] SMU is initialized successfully!
[ 3.961856] [drm] kiq ring mec 2 pipe 1 q 0
[ 3.961980] [drm] ring test on 10 succeeded in 65 usecs
[ 3.962029] [drm] ring test on 10 succeeded in 11 usecs
[ 3.962103] [drm] gfx 0 ring me 0 pipe 0 q 0
[ 3.962123] [drm] ring test on 0 succeeded in 8 usecs
[ 3.962125] [drm] gfx 1 ring me 0 pipe 1 q 0
[ 3.962132] [drm] ring test on 1 succeeded in 1 usecs
[ 3.962134] [drm] compute ring 0 mec 1 pipe 0 q 0
[ 3.962146] [drm] ring test on 2 succeeded in 3 usecs
[ 3.962147] [drm] compute ring 1 mec 1 pipe 1 q 0
[ 3.962161] [drm] ring test on 3 succeeded in 1 usecs
[ 3.962163] [drm] compute ring 2 mec 1 pipe 2 q 0
[ 3.962178] [drm] ring test on 4 succeeded in 1 usecs
[ 3.962179] [drm] compute ring 3 mec 1 pipe 3 q 0
[ 3.962194] [drm] ring test on 5 succeeded in 1 usecs
[ 3.962195] [drm] compute ring 4 mec 1 pipe 0 q 1
[ 3.962210] [drm] ring test on 6 succeeded in 1 usecs
[ 3.962211] [drm] compute ring 5 mec 1 pipe 1 q 1
[ 3.962226] [drm] ring test on 7 succeeded in 1 usecs
[ 3.962227] [drm] compute ring 6 mec 1 pipe 2 q 1
[ 3.962242] [drm] ring test on 8 succeeded in 1 usecs
[ 3.962243] [drm] compute ring 7 mec 1 pipe 3 q 1
[ 3.962258] [drm] ring test on 9 succeeded in 1 usecs
[ 3.962356] [drm] ring test on 11 succeeded in 37 usecs
[ 3.962377] [drm] ring test on 12 succeeded in 5 usecs
[ 3.989875] [drm] VCN decode and encode initialized successfully(under DPG Mode).
[ 3.989986] amdgpu 0000:03:00.0: ring 0(gfx_0.0.0) uses VM inv eng 4 on hub 0
[ 3.989988] amdgpu 0000:03:00.0: ring 1(gfx_0.1.0) uses VM inv eng 5 on hub 0
[ 3.989990] amdgpu 0000:03:00.0: ring 2(comp_1.0.0) uses VM inv eng 6 on hub 0
[ 3.989992] amdgpu 0000:03:00.0: ring 3(comp_1.1.0) uses VM inv eng 7 on hub 0
[ 3.989993] amdgpu 0000:03:00.0: ring 4(comp_1.2.0) uses VM inv eng 8 on hub 0
[ 3.989995] amdgpu 0000:03:00.0: ring 5(comp_1.3.0) uses VM inv eng 9 on hub 0
[ 3.989997] amdgpu 0000:03:00.0: ring 6(comp_1.0.1) uses VM inv eng 10 on hub 0
[ 3.989998] amdgpu 0000:03:00.0: ring 7(comp_1.1.1) uses VM inv eng 11 on hub 0
[ 3.990000] amdgpu 0000:03:00.0: ring 8(comp_1.2.1) uses VM inv eng 12 on hub 0
[ 3.990001] amdgpu 0000:03:00.0: ring 9(comp_1.3.1) uses VM inv eng 13 on hub 0
[ 3.990003] amdgpu 0000:03:00.0: ring 10(kiq_2.1.0) uses VM inv eng 14 on hub 0
[ 3.990005] amdgpu 0000:03:00.0: ring 11(sdma0) uses VM inv eng 15 on hub 0
[ 3.990006] amdgpu 0000:03:00.0: ring 12(sdma1) uses VM inv eng 16 on hub 0
[ 3.990008] amdgpu 0000:03:00.0: ring 13(vcn_dec) uses VM inv eng 4 on hub 1
[ 3.990010] amdgpu 0000:03:00.0: ring 14(vcn_enc0) uses VM inv eng 5 on hub 1
[ 3.990011] amdgpu 0000:03:00.0: ring 15(vcn_enc1) uses VM inv eng 6 on hub 1
[ 3.990013] amdgpu 0000:03:00.0: ring 16(vcn_jpeg) uses VM inv eng 7 on hub 1
[ 3.990094] [drm] Initialized amdgpu 3.33.0 20150101 for 0000:03:00.0 on minor 1
[ 5.999221] [drm] ib test on ring 0 succeeded
[ 5.999846] [drm] ib test on ring 1 succeeded
[ 6.000508] [drm] ib test on ring 2 succeeded
[ 6.001142] [drm] ib test on ring 3 succeeded
[ 6.001793] [drm] ib test on ring 4 succeeded
[ 6.002460] [drm] ib test on ring 5 succeeded
[ 6.003114] [drm] ib test on ring 6 succeeded
[ 6.003773] [drm] ib test on ring 7 succeeded
[ 6.004426] [drm] ib test on ring 8 succeeded
[ 6.005088] [drm] ib test on ring 9 succeeded
[ 6.005720] [drm] ib test on ring 10 succeeded
[ 6.006344] [drm] ib test on ring 11 succeeded
[ 6.006995] [drm] ib test on ring 12 succeeded
[ 19.143576] amdgpu 0000:03:00.0: vgaarb: changed VGA decodes: olddecodes=io+mem,decodes=none:owns=none
However, there is never a line indicating that amd framebuffer was initiallized, like:
[ 10.849100] amdgpu 0000:03:00.0: fb0: amdgpudrmfb frame buffer device
We'll see once you blacklist AST.