msm: kgsl: Look up the process struct in process_mem_open()

The pointer to an active process struct stands the risk of going stale
if process_mem_open() is called just before the debugfs dentry is
destroyed.  Instead of storing and using the pointer pass the process pid
to process_mem_open() and do a lookup to ensure that we always get active
processes.

Change-Id: Ic0dedbad3a3b54dc96251663fdf89c466fef3ca5
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>