msm: kgsl: Don't hold process list global mutex in process private create

Don't hold process list global mutex for long. Instead make
use of process specific spin_lock() to serialize access
to process private structure while creating it. Holding
process list global mutex could lead to deadlocks as other
functions depend on it.

CRs-fixed: 480732
Change-Id: Id54316770f911d0e23384f54ba5c14a1c9113680
Signed-off-by: Harsh Vardhan Dwivedi <hdwivedi@codeaurora.org>
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>