drm/nouveau/sw: fix oops if gpu has its display block disabled

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
diff --git a/drivers/gpu/drm/nouveau/core/engine/software/nv50.c b/drivers/gpu/drm/nouveau/core/engine/software/nv50.c
index b574dd4..5ce686e 100644
--- a/drivers/gpu/drm/nouveau/core/engine/software/nv50.c
+++ b/drivers/gpu/drm/nouveau/core/engine/software/nv50.c
@@ -176,7 +176,7 @@
 	if (ret)
 		return ret;
 
-	chan->vblank.nr_event = pdisp->vblank->index_nr;
+	chan->vblank.nr_event = pdisp ? pdisp->vblank->index_nr : 0;
 	chan->vblank.event = kzalloc(chan->vblank.nr_event *
 				     sizeof(*chan->vblank.event), GFP_KERNEL);
 	if (!chan->vblank.event)