FYI, I tested this our a month or so ago, with build of the kernel / mesa / etc from git and was never able to get it to to work. I've also just re-tested with Fedora rawhide, and am seeing the same behaviour.
Effectively, the graphics card is detected fine, however no output ports are detected when starting X11, and as a result, no screens are found.
Normally, I'd expect to see something like the following in the X11 log:
[ 716.370] (II) AMDGPU(0): Output DisplayPort-0 has no monitor section
[ 716.370] (II) AMDGPU(0): Output DisplayPort-1 has no monitor section
[ 716.370] (II) AMDGPU(0): Output DisplayPort-2 has no monitor section
[ 716.371] (II) AMDGPU(0): Output HDMI-A-0 has no monitor section
[ 716.404] (II) AMDGPU(0): EDID for output DisplayPort-0
With Navi 10 on rawhide, I instead see the following (no outputs type are even detected, so it doesn't probe them):
[ 1002.413] (II) AMDGPU(0): glamor X acceleration enabled on AMD NAVI10 (DRM 3.35.0, 5.4.0-2.fc32.ppc64le, LLVM 9.0.0)
[ 1002.413] (II) AMDGPU(0): glamor detected, initialising EGL layer.
[ 1002.413] (==) AMDGPU(0): TearFree property default: auto
[ 1002.413] (==) AMDGPU(0): VariableRefresh: disabled
[ 1002.413] (II) AMDGPU(0): KMS Pageflipping: enabled
[ 1002.413] (WW) AMDGPU(0): No outputs definitely connected, trying again...
[ 1002.413] (WW) AMDGPU(0): Unable to find connected outputs - setting 1024x768 initial framebuffer
[ 1002.413] (II) AMDGPU(0): mem size init: gart size :1fe810000 vram size: s:1f7b70000 visible:fd50000
[ 1002.413] (==) AMDGPU(0): DPI set to (96, 96)
[ 1002.413] (==) AMDGPU(0): Using gamma correction (1.0, 1.0, 1.0)
...
Fatal server error:
[ 1002.416] (EE) no screens found(EE)
So, my assumption right now is that the current code has a bug on ppc64 where outputs ports are not detected properly. Note that I'll do some additional tests this weekend, but I expect this will require some sort of fix changes in the kernel/amdgpu driver.