include/linux: add low power mode notifier

Introduce low power mode so that listeners are aware when display is
going into low power mode. This requires moving notifier inside sde code
to be able to retrieve the low power state.

Change-Id: I8671e86af29c959d97e1b8ad758a07719ffa4bec
(cherry picked from commit c4b15f56dd63d24d6f4482deab2423809c700d7a)
Signed-off-by: Adrian Salido <salidoa@google.com>
[samtran@codeaurora.org: move changes to different file as
downstream use of notifier struct comes from different file]
Signed-off-by: Samantha Tran <samtran@codeaurora.org>
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>