[media] v4l: vsp1: Base link creation on availability of entities

Check the entity pointer instead of the feature flag to see if the
entity is available before creating related links. The two methods are
currently equivalent, but will differ in the future as we implement
support for ignoring some of the entities present in the hardware.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>