CVE-2026-68261 drm/imagination: fix error checking of pvr_vm_context_lookup()

In the Linux kernel, the following vulnerability has been resolved: drm/imagination: fix error checking of pvrvmcontextlookup Since pvrvmcontextlookup returns either NULL or a pointer, then stop using ISERR for checking the return value. Using ISERR leads to the kernel oops reported below. It can...