[media] V4L: soc-camera: add helper functions for videobuf queue handling

Add two helper inline functions to retrieve soc-camera device context
from videobuf and videobuf2 queue pointers.

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