msm: kgsl: guard all mem_idr operations with the mem_lock

It isn't possible to use rcu_read_lock() sections to guard
access to a data structure that is refcounted with a kref.
Rather than creating RCU-aware refcounts for kgsl_mem_entry
as described in Documentation/RCU/rcuref.txt, just use
the mem_lock to guard lookups in the idr.

Change-Id: Ia0733b156fc7a9b446cb8221b9172ce9faf111e7
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>