media: rcar-vin: Refactor link notify

The code has grown organically and a lot of checks are performed for
the CSI-2 use-case even if the link notify is for a subdevice connected
to the parallel interface.

Before reworking the CSI-2 routing logic split the CSI-2 and parallel
link notify code in two separate blocks to make it clearer. There is no
functional change.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Tested-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>