[media] soc-camera: Add soc_mbus_image_size

The function returns the minimum size of an image for a given number of
bytes per line (as per the V4L2 specification), width and format.

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