msm: kgsl: Fix a race condition when destroying fence events

Make sure the fence event structure is valid when cancelling it,
and that only one of the callback and cancel functions can run. The
function that runs cleans up the associated structures.

CRs-fixed: 2077454
Change-Id: I7c250868ae4fd193aef0424c7d66283a3e7f8121
Signed-off-by: Lynus Vaz <lvaz@codeaurora.org>