videobuffer: fix memory leak for surface and image.

videobuffer: fix memory leak for surface and image.

Fix reference leak of surface and image in GstVaapiVideoBuffer wrapper,
thus resulting on actual memory leak of GstVaapiImage when using them
for downloads/uploads from VA surfaces and more specifically surfaces
when the pipeline is shutdown. i.e. vaTerminate() was never called
because the resources were not unreferenced, and thus not deallocated
in the end.

Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>