drm/nouveau: check for dead channel before trying to idle

This prevents *very* long waits while attempting to destroy channels
after a fault has occurred.

Signed-off-by: Ben Skeggs <[e-mail]>

drivers/gpu/drm/nouveau/nouveau_chan.c

···
 int
 nouveau_channel_idle(struct nouveau_channel *chan)
 {
-	if (likely(chan && chan->fence)) {
+	if (likely(chan && chan->fence && !atomic_read(&chan->killed))) {
 		struct nouveau_cli *cli = (void *)chan->user.client;
 		struct nouveau_fence *fence = NULL;
 		int ret;