msm: kgsl: Changing to kref_get_unless_zero for process refcount

Do not allow anything to get a refcount to the process_private
structure if its refcount has gone down to 0.

Change-Id: I7cd6f731286aebfc84c288284599b47f666dcd47
Signed-off-by: Shrenuj Bansal <shrenujb@codeaurora.org>