v4l: vsp1: Store source and sink pointers as vsp1_entity

The internal VSP entity source and sink pointers are stored as
media_entity pointers, which are then cast to a vsp1_entity. As all
sources and sinks are vsp1_entity instances, we can store the
vsp1_entity 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>