msm: kgsl: Get reference to process private when attaching mem entry

When a memory entry is attached with to a process private list then
increment the refcount of the process private structure so that the
structure is valid as long as the entry is around. The context
structure is also handled in similar manner. Also, the refcount to
the entry needs to be decremented when the handle that created the
entry goes away and not when the process private structure is
destroyed.

Change-Id: I225698ad4081947a93eb553104e5259bbf31f293
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>