drm/nv50/kms: prevent lockdep false-positive in page flipping path

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
diff --git a/drivers/gpu/drm/nouveau/nv50_display.c b/drivers/gpu/drm/nouveau/nv50_display.c
index 2db5799..7f0e6c3 100644
--- a/drivers/gpu/drm/nouveau/nv50_display.c
+++ b/drivers/gpu/drm/nouveau/nv50_display.c
@@ -524,6 +524,8 @@
 	swap_interval <<= 4;
 	if (swap_interval == 0)
 		swap_interval |= 0x100;
+	if (chan == NULL)
+		evo_sync(crtc->dev);
 
 	push = evo_wait(sync, 128);
 	if (unlikely(push == NULL))
@@ -586,8 +588,6 @@
 		sync->addr ^= 0x10;
 		sync->data++;
 		FIRE_RING (chan);
-	} else {
-		evo_sync(crtc->dev);
 	}
 
 	/* queue the flip */