drm/msm/sde: Avoid event notification for SDE power ON case

For command mode, events to user mode is currently doubled
as SDE_POWER_EVENT is giving both ON and OFF events, but
currently HAL has operations defined only for OFF event. As
ON will cause unnecessary event thread wake-ups, make changes
to notify only OFF.

Change-Id: Ic5ac261c41c2999f30a92608d92149362606a5f0
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>