pipeline: simple: Use breadth-first search to setup media pipeline

When the SimplePipeline is setting up its data and media pipeline in the
SimpleCameraData constructor, it merely tries to find the first valid
pad and link to the next entity, starting from the camera sensor.
Following this path may not always lead to a valid capture device and
therefore the setup will fail on some machines. This is for example an
issue when using the SimplePipeline on an i.MX-6Q with its i.MX IPU.

This commit implements a different approach to setup the media-pipeline
by finding the shortest path to a valid capture device, using the
breadth-first search algorithm. On i.MX6Q, the shortest path has a good
chance to be the path from the sensor to the CSI capture device, as
other paths may involve image converters, encoders or other IPU blocks
and will have therefore more nodes.

Signed-off-by: Marian Cichy <m.cichy@pengutronix.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

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