msm: kgsl: Add context reference counting to KGSL events

Add reference counting to all KGSL events since they are dependent
on the context.  This avoids a race condition where the context
could be destroyed while the events are being processed during cleanup.

Change-Id: Ifcd9655a6335d22c01f2606a0fbc5346d9a0314f
CRs-fixed: 438134
Signed-off-by: Carter Cooper <ccooper@codeaurora.org>