[media] v4l: soc-camera: switch to .unlocked_ioctl

Use the V4L mutex infrastructure in soc-camera core and drivers and switch to
.unlocked_ioctl.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>