drm/virtio: move mapping teardown to virtio_gpu_cleanup_object()

Stop sending DETACH_BACKING commands, that will happening anyway when
releasing resources via UNREF.  Handle guest-side cleanup in
virtio_gpu_cleanup_object(), called when the host finished processing
the UNREF command.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200207074638.26386-4-kraxel@redhat.com


Reviewed-by: Chia-I Wu <olvaffe@gmail.com>