drm/radeon: free uvd ring on unload

Need to free the uvd ring. Also reshuffle gart tear down to
happen after uvd tear down.

Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Cc: stable@vger.kernel.org
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>