video: always update the brightness when poking "brightness"

commit 9e6dada9d255497127251c03aaa59296d186f959 upstream.

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>
Cc: Thomas Renninger <trenn@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>