msm: kgsl: Use CACHE_FLUSH_TS for global timestamp

There have been cases where the global timestamp is ahead
of the context timestamp. To remove this anomaly, use the
EVENT_WRITE packet instead of MEM_WRITE.

Change-Id: Ib8243949796a0243cbf5e343b4b1443cc7b7c1d3
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>