video: always update the brightness when poking "brightness"

always update props.brightness no matter the backlight is changed
via procfs, hotkeys or sysfs.

Sighed-off-by: Zhang Rui <rui.zhang@intel.com>
Acked-by: Matthew Garrett <mjg59@srcf.ucam.org>
Signed-off-by: Len Brown <len.brown@intel.com>