msm: kgsl: Prevent race condition when freeing memory

Add a check to set the pending_free flag if it is not already
set before freeing sparse memory entry. This is required to
prevent multiple ioctl threads from freeing the same sparse
memory entry.

Change-Id: I4e2bbe6fcd98c58d36340c4f87cdff27fc1de22e
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>