Add a V4L2Device for the default video entity in the media graph. The UVC pipeline needs to search for the entity marked with the MEDIA_ENT_FL_DEFAULT flag as the entity names in the media graph varies depending on which device is used. Signed-off-by: Niklas Söderlund <niklas.soderlund@ragnatech.se> Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>