backlight: backlight: Supply description for function args in existing Kerneldocs

Kerneldoc syntax is used, but not complete.  Descriptions required.

Prevents warnings like:

 drivers/video/backlight/backlight.c:329: warning: Function parameter or member 'reason' not described in 'backlight_force_update'
 drivers/video/backlight/backlight.c:354: warning: Function parameter or member 'props' not described in 'backlight_device_register'

Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Jamey Hicks <jamey.hicks@hp.com>
Cc: Andrew Zabolotny <zap@homelink.ru>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>