CVE-2026-98514: drm/nouveau: Fix gem reference leak in validate_init()

In the Linux kernel, the following vulnerability has been resolved:

drm/nouveau: Fix gem reference leak in validateinit()

On the ttmboreserve() failure and "vma not found" error paths, the loop breaks without adding the looked-up object to any validate list, so the reference taken by drmgemobjectlookup() is never released; validatefini() only walks the spliced lists. Drop the reference before breaking out on both paths.