msm: mdss: Turn off backlight while power off

Turn off the backlight while shut down panel as it
could show white flicker depending on the panel.

Change-Id: Ic5d4e97e70c87e5b6e956e27a766136610d6f345
Signed-off-by: Shuo Yan <shuoy@codeaurora.org>
Signed-off-by: Team-CodeCraft <kernelcraft@gmail.com>