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>

drivers/gpu/drm/nouveau/nouveau_drv.c

@@ -277,7 +277,7 @@ nouveau_pci_resume(struct pci_dev *pdev)
 
 		for (i = 0; i < dev_priv->engine.fifo.channels; i++) {
 			chan = dev_priv->fifos[i];
-			if (!chan)
+			if (!chan || !chan->pushbuf_bo)
 				continue;
 
 			for (j = 0; j < NOUVEAU_DMA_SKIPS; j++)