drm/amdgpu: enable DPG mode for VCN3.0

Enable DPG mode for VCN3.0 by updating related flag.

V2: update description.

Signed-off-by: Boyuan Zhang <boyuan.zhang@amd.com>
Reviewed-by: James Zhu <james.zhu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>