[media] v4l: Allow changing control handler lock

Allow choosing the lock used by the control handler. This may be handy
sometimes when a driver providing multiple subdevs does not want to use
several locks to serialise its functions.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>