[media] v4l: omap4iss: csi2: Perform real frame number propagation

Compute the pipeline frame number from the frame number sent by the
sensor instead of incrementing the frame number in software. This
improves dropped frames detection.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>