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>
msm: mdss: disable irq to enforce mutex within dsi_event_thread

Mdp spin lock is used in dsi_event_thread and in irq context.
Disable irq while holding spinlock to avoid context switching
into irq context and cause spinlock recursion

CRs-fixed: 652850
Change-Id: I4bd8cadb1f5d83acc7e866344b7585e7944e08ac
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>
Add missing var