v4l: vsp1: Don't create links for DRM pipeline

When the VSP1 is used in a DRM pipeline the driver doesn't register the
media device. Links between entities are not exposed to userspace, but
are still used internally for the sole purpose of setting up internal
source to sink pointers through the link setup handler.

Instead of going through this complex procedure, remove link creation
and set the sink pointers directly.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Acked-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>