[media] v4l: vsp1: Change the type of the rwpf field in struct vsp1_video

The rwpf field contains a pointer to the rpf or wpf associated with the
video node. Instead of storing it as a vsp1_entity, store the
corresponding vsp1_rwpf pointer to allow accessing the vsp1_rwpf fields
directly.

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