drm/cma-helper: fixup compilation

/me grabs a few brown paper bags

So it looks like I've broken compilation in

commit 6aed8ec3
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Sun Jan 20 17:32:21 2013 +0100

    drm: review locking for drm_fb_helper_restore_fbdev_mode

Fix it up again.
Reported-by: NWu Fengguang <fengguang.wu@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_fb_cma_helper.c

@@ -377,6 +377,8 @@ EXPORT_SYMBOL_GPL(drm_fbdev_cma_fini);
  */
 void drm_fbdev_cma_restore_mode(struct drm_fbdev_cma *fbdev_cma)
 {
+	struct drm_device *dev = fbdev_cma->fb_helper.dev;
+
 	drm_modeset_lock_all(dev);
 	if (fbdev_cma)
 		drm_fb_helper_restore_fbdev_mode(&fbdev_cma->fb_helper);