drm/amd/pm: enable MP0 DPM for sienna_cichlid

Enable MP0 clock DPM for sienna_cichlid.

Signed-off-by: Jiansong Chen <Jiansong.Chen@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>