this page:
https://wiki.gentoo.org/wiki/AMDGPU suggests amdgpu/dimgrey_cavefish_{ce,me,mec2,rlc,smc,ta,dmcub,mec,pfp,sdma,sos,vcn}.bin, which i've tried and never gotten to work. Maybe it's because my firmware is out of date. A guy on IRC told me you should just check your petitboot's dmesg (you can still do this over serial if your VGA is disabled with the jumper) and see what firmware files it complains about, which may be more than just the ones strictly necessary for your card.