drm/amdgpu/mmhub: add clockgating support for vega12

Treat it the same as vega10 for now.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>