[media] v4l: vsp1: Move vsp1_video pointer from vsp1_entity to vsp1_rwpf

Only RPFs and WPFs can be associated with video nodes, don't waste
memory by storing the video pointer in all entities.

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