msm: kgsl: Fix memleak in IOMMU clock disable

IOMMU clock disable on timestamp allocates memory and passes
the pointer to event created, this memory should be freed
when the event fires.

Change-Id: Ia714550ce8a05958fb6668293cd0400a0f69d4af
Signed-off-by: Tarun Karra <tkarra@codeaurora.org>