msm: kgsl: Fix creating invalid context when process private get fails

kgsl_context_init() returns 0 (success) even if getting
reference to process private fails, with this we will end up
creating an invalid context. Return error when process
refcounting fails to avoid creating invalid context.

Change-Id: Iad6a55c48f42c435027013615fab675238247be8
Signed-off-by: Prakash Kamliya <pkamliya@codeaurora.org>