msm: kgsl: fix kgsl_mem_entry refcounting

Make kgsl_sharedmem_find* return a reference to the
entry that was found. This makes using an entry
without the mem_lock held less race prone.

Change-Id: If6eb6470ecfea1332d3130d877922c70ca037467
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>