msm: kgsl: make stats updation atomic

There is a chance of messing up per process stats if something
like a free and allocation happen at the same time for memory
belonging to the same process. Similarly mapsize of memdesc can
also go bad because of concurrent access. So make per process
stats and mapsize of memdesc atomic.

Change-Id: I5da1cf368d523768d0b267b314394612934a620d
Signed-off-by: Puranam V G Tejaswi <pvgtejas@codeaurora.org>