msm: kgsl: fix race condition setting the event->handle

A race condition existed for setting the event->handle since the
async event can signal at any time after it has been registered.
When the event signals the reference count for the synclist and
the async callback are decremented causing the event to be freed.
The kgsl_cmdbatch_add_sync_fence function needs to take another
reference to ensure that the handle can be set before the event
is freed.

Change-Id: I0d8a02246e42dce014661d6f14c685415f1704cd
Signed-off-by: Jeff Boody <jboody@codeaurora.org>