drm/nouveau/fence: convert to exec engine, and improve channel sync

Now have a somewhat simpler semaphore sync implementation for nv17:nv84,
and a switched to using semaphores as fences on nv84+ and making use of
the hardware's >= acquire operation.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_debugfs.c

@@ -67,8 +67,6 @@ nouveau_debugfs_channel_info(struct seq_file *m, void *data)
 			   nvchan_rd32(chan, 0x8c));
 	}
 
-	seq_printf(m, "last fence    : %d\n", chan->fence.sequence);
-	seq_printf(m, "last signalled: %d\n", chan->fence.sequence_ack);
 	return 0;
 }