drm/nv50: fix two potential suspend/resume oopses

This avoids touching the dummy channel 0/127 we have on nv50.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
diff --git a/drivers/gpu/drm/nouveau/nv50_fifo.c b/drivers/gpu/drm/nouveau/nv50_fifo.c
index 77ae1aa..b728228 100644
--- a/drivers/gpu/drm/nouveau/nv50_fifo.c
+++ b/drivers/gpu/drm/nouveau/nv50_fifo.c
@@ -416,7 +416,7 @@
 	NV_DEBUG(dev, "\n");
 
 	chid = pfifo->channel_id(dev);
-	if (chid < 0 || chid >= dev_priv->engine.fifo.channels)
+	if (chid < 1 || chid >= dev_priv->engine.fifo.channels - 1)
 		return 0;
 
 	chan = dev_priv->fifos[chid];