staging: vboxvideo: Call fb_deferred_io_cleanup() on cleanup

On driver cleanup we need to call fb_deferred_io_cleanup() if build
with CONFIG_FB_DEFERRED_IO set.

Suggested-by: Michael Thayer <michael.thayer@oracle.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>