drm/i915: take mode config lock around crtc disable at suspend

This is just a theoretical issue, but we need to do this to prevent the
WARN in pipe_from_connector at suspend time.

This regression has been introduce in

commit 7bd688cd
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Fri Nov 8 16:48:56 2013 +0200

    drm/i915: handle backlight through chip specific functions

https://bugs.freedesktop.org/show_bug.cgi?id=71978


Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>