[media] vsp1: Fix lack of the sink entity registration for enabled links

Each source entity maintains a pointer to the counterpart sink
entity while an enabled link connects them. It should be managed by
the setup_link callback in the media controller framework at runtime.
However, enabled links which connect RPFs and WPFs that have an
equivalent index number are created during initialization.
This registers the pointer to a sink entity from the source entity
when an enabled link is created.

Signed-off-by: Katsuya Matsubara <matsu@igel.co.jp>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>