msm: kgsl: hold a process_private refcount in process_mem_print

There is a possible race condition where the process can be going
away while its debugfs 'mem' file is being read, which could cause
memory corruption.

CRs-Fixed: 627780
Change-Id: I697486faeb3f186fd1220d0acc1e449a4f7b77b0
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>