[media] v4l: vsp1: Store active formats in a pad config structure

Add a pad config structure field to the vsp1_entity structure and use it
to store all active pad formats. This generalizes the code to operate on
pad config structures.

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