disp: msm: sde: update last power event handled correctly

Update triggered power event to last event correctly,
regardless of whether a callback is called.
Add event log to see debugfs clock rate change.

Change-Id: Ifa9c1ffb450f50a3928eb44362723b6d495b2354
Signed-off-by: Nilaan Gunabalachandran <ngunabal@codeaurora.org>