msm: mdss: Add support to update backlight when display commit

1. Add support to update backlight when display commit. Backlight
level is sent from userspace. And sync to set PP config, kickoff,
and set backlight in the same vsync period.

2. Add support to get current backlight level through sysfs node.

Change-Id: Icf264299ecf8edd3831938745a471c2655c23542
Signed-off-by: Xu Yang <yangxu@codeaurora.org>