drm/amdgpu: add Sienna_Cichlid VCN PG and CG support (v2)

This is for static powergating and clockgating

v2: fix registers (Alex)
Signed-off-by: Leo Liu <leo.liu@amd.com>
Reviewed-by: James Zhu <James.Zhu@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>