V4L/DVB (10083): soc-camera: unify locking, play nicer with videobuf locking

Move mutex from host drivers to camera device object, take into account
videobuf locking.

Signed-off-by: Guennadi Liakhovetski <lg@denx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>