[media] v4l: vsp1: Replace container_of() with dedicated macro

Add a macro to cast from a struct media_entity to a struct vsp1_entity
to replace the manual implementations.

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