drm/nouveau: Fix WARN_ON in nouveau_fence_context_kill()

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.

Cc: stable@vger.kernel.org # v5.10+
Fixes: ea13e5abf807 ("drm/nouveau: signal pending fences when channel has been killed")
Suggested-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://lore.kernel.org/r/20250415121900.55719-3-phasta@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>