[media] v4l: vsp1: Make vsp1_entity_get_pad_compose() more generic

Turn the helper into a function that can retrieve crop and compose
selection rectangles.

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