V4L/DVB (7501): soc-camera: use a spinlock for videobuffer queue

All drivers should provide a spinlock to be used in videobuf operations.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@pengutronix.de>
Reviewed-by: Brandon Philips <bphilips@suse.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>