media: media-entity: Add media_pad_is_streaming() helper function

Add a function to test if a pad is part of a pipeline currently
streaming, and use it through drivers to replace direct access to the
stream_count field. This will help reworking pipeline start/stop without
disturbing drivers.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>