msm: kgsl: Verify the pointer isn't NULL before using it for kref

Probably overkill, but ensure that the struct pointer we are going
to dereference to send into kref calls is valid before we dereference.

Change-Id: I308176df9f7476a2a9f1357612381a93160ad698
Signed-off-by: Carter Cooper <ccooper@codeaurora.org>