drm/msm/sde: Fix out of bound access in ESD notify event

For event notification, event length is used to perform
copy to notify structure. Use correct event length such
that there is no out of bound access.

Change-Id: I781c8efc34ec506247554c6ab1dd10f912835da6
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>