ANDROID: driver: gpu: drm: add notifier for panel related events

Add support for notfier of panel blank/unblank events.
This allows external drivers such as touch, backlight
drivers etc to subscribe to panel related events.

Signed-off-by: Shashank Babu Chinta Venkata <sbchin@codeaurora.org>
Bug: 139653858
Change-Id: I3ac644c1c931b959a511ee4a999a417a4b7bdcd1