drm/amdgpu: No need support vcn decode

As no need to support vcn decode feature, so disable the
ring for SR-IOV.

Signed-off-by: Emily Deng <Emily.Deng@amd.com>
Reviewed-by: Monk Liu <monk.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>