V4L/DVB (8337): soc_camera: make videobuf independent

Makes SoC camera videobuf independent. Includes all necessary changes for
PXA camera driver (currently the only driver using soc_camera in the mainline).
These changes are important for the future soc_camera based drivers.

Signed-off-by: Paulius Zaleckas <paulius.zaleckas@teltonika.lt>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@pengutronix.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>