drm/amdgpu: Add GFXHUB v1.2 XCP funcs

Add functions required for suspend/resume of GFXHUB instances which are
part of an XCP.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>