video: deferred io cleanup

Make sure the mmap callback is set to NULL in the deferred io
cleanup function. This way we can enable and disable deferred
io on the fly.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Acked-by: Jaya Kumar <jayakumar.lkml@gmail.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>