drm/nouveau: Work around a crash during boot if noaccel is set.

[Ortwin Glück]

NB: still broken in 3.5 as well.

Signed-off-by: Ortwin Glück <odi at odi.ch>
---
diff --git a/drivers/gpu/drm/nouveau/nv50_display.c 
b/drivers/gpu/drm/nouveau/nv50_display.c
index b244d99..c7ffa63 100644
--- a/drivers/gpu/drm/nouveau/nv50_display.c
+++ b/drivers/gpu/drm/nouveau/nv50_display.c
@@ -650,6 +650,12 @@ nv50_display_vblank_crtc_handler(struct drm_device 
*dev, int crtc)
  	struct nouveau_software_priv *psw = nv_engine(dev, NVOBJ_ENGINE_SW);
  	struct nouveau_software_chan *pch, *tmp;

+        if (!psw) {
+                WARN_ON_ONCE(1);
+                printk(KERN_ERR "NULL software engine\n");
+                return;
+        }
+
  	list_for_each_entry_safe(pch, tmp, &psw->vblank, vblank.list) {
  		if (pch->vblank.head != crtc)
  			continue;