[media] soc-camera: remove struct soc_camera_device::video_lock

Currently soc-camera has a per-device node lock, used for video operations
and a per-host lock for code paths, modifying host's pipeline. Manipulating
the two locks increases complexity and doesn't bring any advantages. This
patch removes the per-device lock and uses the per-host lock for all
operations.

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