drm/virtio: track created object state

Track whenever the virtio_gpu_object is already created (i.e. host knows
about it) in a new variable.  Add checks to virtio_gpu_object_attach()
to do nothing on objects not created yet.

Make virtio_gpu_ttm_bo_destroy() use the new variable too, instead of
expecting hw_res_handle indicating the object state.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20181019061847.18958-2-kraxel@redhat.com