drm/msm/sde: provide sde power notification support

Generates sde power on/off custom notification events to usermode.
Usermode driver needs to register for the new custom event
DRM_EVENT_SDE_POWER in order to receive the notification.

Change-Id: Id26862d2fb4ecb77c4906a0426dea6950ae6a955
Signed-off-by: Benjamin Chan <bkchan@codeaurora.org>