drm/amd/amdgpu: add AMD_PG_SUPPORT_VCN_DPG flag

AMD_PG_SUPPORT_VCN_DPG is needed for secure parts
and should/can be enabled by now.

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