msm: kgsl: Handle failure of find_process_private()

Handle the case of NULL return from find_process_private() upon
its failure by failing from get_process_private().

CRs-fixed: 542831
Change-Id: I8a365368ecf68f0d1aa01199aca76bc7ac49e154
Signed-off-by: Harsh Vardhan Dwivedi <hdwivedi@codeaurora.org>