It took me some times, but here is a patch correcting commit
20abd1634a6e2eedb84ca977adea56b8aa06cc3e:
---------------------------------------------------------------------------------

diff --git a/drivers/gpu/drm/nouveau/nouveau_software.h
b/drivers/gpu/drm/nouveau/nouveau_software.h
index fe30a8f..7adfcb9 100644
--- a/drivers/gpu/drm/nouveau/nouveau_software.h
+++ b/drivers/gpu/drm/nouveau/nouveau_software.h
@@ -20,10 +20,10 @@ struct nouveau_software_chan {
 static inline void
 nouveau_software_vblank(struct drm_device *dev, int crtc)
 {
-       struct nouveau_software_priv *psw = nv_engine(dev, NVOBJ_ENGINE_SW);
+       struct drm_nouveau_private *dev_priv = dev->dev_private;
        struct nouveau_software_chan *pch, *tmp;

-       list_for_each_entry_safe(pch, tmp, &psw->vblank, vblank.list) {
+       list_for_each_entry_safe(pch, tmp, &dev_priv->vbl_waiting, vblank.list)
{
                if (pch->vblank.head != crtc)
                        continue;

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(The above empty line is needed)

However, the code was later modified, and the patch can't be applied on recent
kernel; I'll try to get a new patch for it this week-end.