amdgpu/vcn: enable AMD_PG_SUPPORT_VCN

turn on AMD_PG_SUPPORT_VCN flag for power saving

Signed-off-by: David (Ming Qiang) Wu <David.Wu3@amd.com>
Reviewed-by: Sonny Jiang <sonny.jiang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>