media: subdev: Refactor v4l2_subdev_enable/disable_streams()

Add two internal helper functions, v4l2_subdev_collect_streams() and
v4l2_subdev_set_streams_enabled(), which allows us to refactor
v4l2_subdev_enable/disable_streams() functions.

This (I think) makes the code a bit easier to read, and lets us more
easily add new functionality in the helper functions in the following
patch.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Umang Jain <umang.jain@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>