media: v4l2-dev: Add videodev wrappers for media pipelines

With the upcoming stream related improvements to the pipelines, the
pipelines are moved from media entities to media pads. As the drivers
currently use the pipelines with the entity based model, moving the
pipelines to pads will cause changes to the drivers.

However, most of the uses of media pipelines are related to a video
device (a DMA engine) with a single pad, and thus there's never a need
to support multiple pads in these use cases. We can avoid pushing the
complexities of the pad based model to the drivers by adding video
device wrappers for the pipeline related functions.

This patch adds a number of wrappers to media_pipeline functions, all of
which take a video_device as a parameter (instead of a media_entity),
and verify that there's just one pad.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>