drm/amdgpu: add gfxhub v1_2 support

Hack the gc 9.0 reg offset for initial support

v2: squash in header switch (Alex)

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>