msm: kgsl: Improve locking of process list

Use a light weight lock for process list to avoid lock contention.

Change-Id: I8d751c5aa3f61066af097a0c67abbed8c399b913
Signed-off-by: Akhil P Oommen <akhilpo@codeaurora.org>
Signed-off-by: Puranam V G Tejaswi <pvgtejas@codeaurora.org>