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

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nv50_display.c

@@ -524,6 +524,8 @@ nv50_display_flip_next(struct drm_crtc *crtc, struct drm_framebuffer *fb,
 	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 @@ nv50_display_flip_next(struct drm_crtc *crtc, struct drm_framebuffer *fb,
 		sync->addr ^= 0x10;
 		sync->data++;
 		FIRE_RING (chan);
-	} else {
-		evo_sync(crtc->dev);
 	}
 
 	/* queue the flip */