drm/cma-helper: fixup compilation

/me grabs a few brown paper bags

So it looks like I've broken compilation in

commit 6aed8ec3f76a22217c9ae183d32b1aa990bed069
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.

v2: Only deref fbdev_cma once we're sure it's non-NULL, noticed by
Thierry Reding.

Reported-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>