msm: mdss: Add backlight attenuation mechanism in AD auto strength mode

AD backlight attenuation provides the ability to modulate backlight based
on calibration before feeding it to AD core for strength computation.

Change-Id: Iea011807b25882463b1142e17324fa0509007aa4
Signed-off-by: Ping Li <quicpingli@codeaurora.org>