drm/amdgpu/vcn: enable VCN DPG mode for VCN4_0_4

Enable VCN DPG mode for VCN4_0_4.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Sonny Jiang <sonny.jiang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>