drm/amdgpu/gfx9: Add gfx config for vega20. (v4)

v2: clean up (Alex)
v3: additional cleanups (Alex)
v4: drop leftover TODO (Alex)

Signed-off-by: Feifei Xu <Feifei.Xu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>