msm: kgsl: Fix checking of validity of process private pointer

When walking through the list of process private pointers if the
right pointer is not found then process private pointer may not be
NULL. Hence, checking the validity of process private pointer based
on it being NULL is incorrect. Fixed this.

Change-Id: I79bec9dc3fb80342dfc771d427a0f5bad805295f
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>