drm/msm/sde: add back-light notification for AD

Assertive display block generates interrupt when it can attenuate the
back-light. Color processing driver will notify the event through msm
drm custom event interface. Change adds support for the AD back-light
notification.

Change-Id: I3bf461194e9f7e0bf427bd70a1d0b4222a0300db
Signed-off-by: Gopikrishnaiah Anandan <agopik@codeaurora.org>