media: atomisp: Use a normal mutex for the main lock

There is no reason for atomisp to use a rt_mutex instead of a normal
mutex, so switch over to a normal mutex.

All the changes in this patch are just s/rt_mutex/mutex/.

This is a preparation patch for switching the ioctl locking over
to using the video_dev.lock member so that the v4l2-core takes
care of the locking.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>