msm: mdss: Correctly reset the ad_events count

Reset the ad_events count in the AD ioctl instead of ad_setup().
There are some time delay between AD ioctl and ad_setup, which is
part of the display commit path, so if we reset the ad_event count
in ad_setup, then it is possible that user space reads the ad_events
sysfs node after the AD ioctl but before the ad_setup(). In that
case, user space may get the incorrect ad_events count, and cause
the gradual AD off fail.

Change-Id: I13f2043a6a21be7e49b44a964b00b0aa4315dc17
Signed-off-by: Ping Li <pingli@codeaurora.org>