staging: vboxvideo: unlock on error in vbox_cursor_atomic_update()

We need to unlock before returning on this error path.

Fixes: 35f3288c ("staging: vboxvideo: Atomic phase 1: convert cursor to universal plane")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>