drm/nouveau: fix lock unbalance in nouveau_crtc_page_flip

Fixes a regression introduced by d5c1e84b
"drm/nouveau: hold mutex while syncing to kernel channel".

Cc: stable@vger.kernel.org #3.13
Reported-by: NFengguang Wu <fengguang.wu@intel.com>
Signed-off-by: NMaarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_display.c

@@ -722,7 +722,7 @@ nouveau_crtc_page_flip(struct drm_crtc *crtc, struct drm_framebuffer *fb,
 	ret = nouveau_fence_sync(fence, chan);
 	nouveau_fence_unref(&fence);
 	if (ret)
-		goto fail_free;
+		goto fail_unpin;
 
 	ret = ttm_bo_reserve(&old_bo->bo, true, false, false, NULL);
 	if (ret)