[media] v4l: Check pad number in get try pointer functions

Unify functions to get try pointers and validate the pad number accessed by
the user.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>