media: v4l2-subdev: Add v4l2_subdev_s_stream_helper() function

The v4l2_subdev_s_stream_helper() helper can be used by subdevs that
implement the stream-aware .enable_streams() and .disable_streams()
operations to implement .s_stream(). This is limited to subdevs that
have a single source pad.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>