drm/nouveau: fix regression causing ttm to not be able to evict vram

TTM assumes an error condition from man->func->get_node() means that
something went horribly wrong, and causes it to bail.

The driver is supposed to return 0, and leave mm_node == NULL to
signal that it couldn't allocate any memory.

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

drivers/gpu/drm/nouveau/nouveau_mem.c

···
 	ret = vram->get(dev, mem->num_pages << PAGE_SHIFT,
 			mem->page_alignment << PAGE_SHIFT, size_nc,
 			(nvbo->tile_flags >> 8) & 0xff, &node);
-	if (ret)
-		return ret;
+	if (ret) {
+		mem->mm_node = NULL;
+		return (ret == -ENOSPC) ? 0 : ret;
+	}
 
 	node->page_shift = 12;
 	if (nvbo->vma.node)

drivers/gpu/drm/nouveau/nouveau_mm.c

···
 		return 0;
 	}
 
-	return -ENOMEM;
+	return -ENOSPC;
 }
 
 int