drm/msm/sde: Modify event notifier size to overcome out of bounds errors

In DRM event notifier size of event is currently caculated
based on response event which is causing out of bounds access.
This change modifies this calculation based on drm
event instead of response event.

Change-Id: I67e0fd0da076313afcf4bfd347338400b612430d
Signed-off-by: Narendra Muppalla <NarendraM@codeaurora.org>