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.

CRs-fixed: 438134
Change-Id: Ic2cd484d441e4c392ab63b0a0131accaceafc711
Signed-off-by: Carter Cooper <ccooper@codeaurora.org>
(cherry picked from commit 9e8aae1d56e1cf05f19d95d027d87d8aba6e791a)