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.

Change-Id: I4f60737393efa2001597f0222aa6fe19fb6a07c5
Signed-off-by: Harsh Vardhan Dwivedi <hdwivedi@codeaurora.org>
Signed-off-by: Ananta Kishore K <akollipa@codeaurora.org>