msm: mdss: Add absolute backlight control for assertive display

Allow for assertive display to disable backlight updates (except for
updates to turn off display) as an option for automatic backlight mode. If
backlight updates are not prevented, flickering can occur when automatic
backlight mode is enabled and system tries to update backlight value due to
assertive display block controlling backlight values.

Change-Id: I250167e3387e3fbc795f7dfe9fc5f20398d6ca8c
Signed-off-by: Carl Vanderlip <carlv@codeaurora.org>