msm: kgsl: Store process private pointer in context

Store the process private pointer in the context. Earlier the process
private pointer was referenced through the dev_priv pointer in the
context, but the dev_priv pointer can be destroyed before the context
process private so store this pointer locally.

Change-Id: Ic07680b79db55d6306306bd61bda5a1288813914
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>