drm/virtio: resource teardown tweaks

Add new virtio_gpu_cleanup_object() helper function for object cleanup.
Wire up callback function for resource unref, do cleanup from callback
when we know the host stopped using the resource.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200207074638.26386-3-kraxel@redhat.com


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