drm/msm: add event support in sde power handle

Add MDP core clocks enable/disable events support in
sde power handler. Client can request for multiple
events with a single register events call.

Change-Id: Icaa0fffc555e6e0d2dc34916eb03a70ed72bc857
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>