drm/amdgpu: add mmhub v1_8 support

Hack the mmhub 1.7 reg offset for initial support

v2: squash in header switch, CG funcs (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>