drm/amd/pp: Add smu support for VCN powergating on RV

Add the powerplay callback for powergating VCN (same
as UVD and VCE).

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>