msm: kgsl: change timestamp frees to use kgsl_event

The timestamp memqueue was unsorted, which could cause
memory to not be freed soon enough. The kgsl_event
list is sorted and does almost exactly the same thing
as the memqueue did, so freememontimestamp is now
implemented using the kgsl_event list.

CRs-Fixed: 327647
Change-Id: Ia93431bc5f31a9286cc78a92271b43a3a6ac9baf
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>