media: subdev: Support privacy led in v4l2_subdev_enable/disable_streams()

We support camera privacy leds with the .s_stream() operation, in
call_s_stream(), but we don't have that support when the subdevice
implements .enable/disable_streams() operations.

Add the support by enabling the led when the first stream for a
subdevice is enabled, and disabling the led then the last stream is
disabled.

Reviewed-by: Umang Jain <umang.jain@ideasonboard.com>
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>