msm: mdss: Notify AD only when brightness changes

Previously, any backlight change to panel will trigger assertive
display (AD) feature to conduct screen refresh calls. This will
trigger unnecessary screen refresh if AD is enabled together with
CABL feature. This change modifies the backlight notification
mechanism to notify AD only when the Android brightness level
is changed.

Change-Id: I5647269b9651d3a17d896e4047f01b7a244581a4
Signed-off-by: Ping Li <pingli@codeaurora.org>