msm: kgsl: Fix memory leak in _insert_gpuaddr()

There is a memory leak in _insert_gpuaddr() when a duplicate address
entry is found in the rb tree. Fix this by calling kmem_cache_free()
on the allocated memory.

Change-Id: Ia492d7cc523775bacc8a1fcfcd275fe922466e0b
Signed-off-by: Akhil P Oommen <akhilpo@codeaurora.org>