msm: kgsl: Fix the process sysfs refcounting

The kobject used for the process's sysfs node did not provide a
release function, so it did not correctly implement the refcounting.
Add a release callback so that we keep the process structure valid
as long as the kobject is alive.

Change-Id: I6db54092ed29ecd4d2f157188a4f1a5fc70f1edf
Signed-off-by: Lynus Vaz <lvaz@codeaurora.org>