drm/amdgpu: move the VCN DPG mode read and write to VCN

Since this is VCN specific and only used by VCN

Signed-off-by: Leo Liu <leo.liu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>