drm/virtio: add worker for object release

Move object release into a separate worker.  Releasing objects requires
sending commands to the host.  Doing that in the dequeue worker will
cause deadlocks in case the command queue gets filled up, because the
dequeue worker is also the one which will free up slots in the command
queue.

Reported-by: Chia-I Wu <olvaffe@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Tested-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190830060116.10476-1-kraxel@redhat.com