drm/amdgpu: Enable Multi Media Hub (MMHUB) Clock Gating for sienna_cichlid.

Enable mmhub clockgating.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Kenneth Feng <kenneth.feng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>