msm: kgsl: Make sure to synchronize RCUs before freeing sync events

We are freeing cmdbatch sync events before synchronizing the RCUs
which results in the list being out of date while the command batch
is being destroyed and accessing freed memory.

Change-Id: Ic0dedbadfb96d3ca14a6adaa015d7ff26c760c36
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>