msm: mdss: Add update notify for backlight change in static screen

Previously the change of backlight sysfs node value will not trigger
notify updates if display is in static screen case. This will cause
delay or wrong configuration for assertive display. This patch fixes
the above issue by adding an update notify if backlight changes when
there are no display updates.

Change-Id: I1475896214a50dc1fa71739aa751b2e626d27adb
Signed-off-by: Ping Li <quicpingli@codeaurora.org