media: rcar-vin: Link parallel input media entities

When running with media-controller link the parallel input
media entities with the VIN entities at 'complete' callback time.

To create media links the v4l2_device should be registered first.
Check if the device is already registered, to avoid double registrations.

Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Acked-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>