media: v4l2-subdev: Add subdev .(enable|disable)_streams() operations

Add two new subdev pad operations, .enable_streams() and
.disable_streams(), to allow control of individual streams per pad. This
is a superset of what the video .s_stream() operation implements.

To help with handling of backward compatibility, add two wrapper
functions around those operations, and require their usage in drivers.

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