drm: msm: Add support AD4 custom event notifications

SDM845 target supports AD4 feature. User-space client of AD4 driver
needs to be notified of system back-light notifications, suspend/resume
events. Change adds support in sde display driver for notify the events.

Change-Id: Ia2fbae75cc66389145292082c46d3bd16653e941
Signed-off-by: Gopikrishnaiah Anandan <agopik@codeaurora.org>