drm/nouveau: Fix WARN_ON in nouveau_fence_context_kill()

In the Linux kernel, the following vulnerability has been resolved: drm/nouveau: Fix WARN_ON in nouveau_fence_context_kill() Nouveau is mostly designed in a way that it's expected that fences only ever get signaled through nouveau_fence_signal(). However, in at least one other place, nouveau_fence_done(), can signal fences, too. If that happens (race) a signaled fence remains in the pending list for a while, until it gets removed by nouveau_fence_update(). Should nouveau_fence_context_kill() run in the meantime, this would be a bug because the function would attempt to set an error code on an already signaled fence. Have nouveau_fence_context_kill() check for a fence being signaled.