msm: kgsl: Return mementry only if pending_free is not set

Return mementry from kgsl_sharedmem_find only if pending_free
is not set for that mementry. This is necessary to avoid use
of a mementry after it is already marked for free.

Change-Id: I23111e9c82a88ccbda2ab259074c38d91f9ff5cb
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>