msm: kgsl: Clear the memstore while destroying the context

When creating a context, we add the event group much before
initializing the memstore for that context. Between these events,
its possible that events are registered and retired and the
timestamp read in retire_events() gets us the last timestamp of
the last destroyed context. This results in the processed
timestamp to be greater than the actual retired timestamp in the
memstore which is very problematic for us.

CRs-Fixed: 640550
Change-Id: I2ace6d99e2ce417ba38f6bbbeeb787478eb4e372
Signed-off-by: Shrenuj Bansal <shrenujb@codeaurora.org>