media: subdev: add "opposite" stream helper funcs

Add two helper functions to make dealing with streams easier:

v4l2_subdev_routing_find_opposite_end - given a routing table and a pad
+ stream, return the pad + stream on the opposite side of the subdev.

v4l2_subdev_state_get_opposite_stream_format - return a pointer to the
format on the pad + stream on the opposite side from the given pad +
stream.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>