staging: vboxvideo: Drop vbox_bo_unref() helper

Drop the unnecessary vbox_bo_unref() helper and directly call ttm_bo_put()
at the single call site.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>