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: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>