msm: kgsl: fix a race condition when calling kref_put

Fix a race condition which can occur if a thread tries to acquire
a reference to a mem_entry or context while another thread has
already decremented the refcount to 0 and is in the process of
destroying said mem_entry or context.

Change-Id: I6be64ca75f9cb12b03e870b9ca83588197c64e5e
Signed-off-by: Shrenuj Bansal <shrenujb@codeaurora.org>