media: atomisp: Use video_dev.lock for ioctl locking

Set video_dev.lock to point to isp->mutex so that the core does
the locking surroundig ioctls for us and drop all the now no longer
necessary (and conflicting) locking from the ioctl handling code.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>