drm/i915: Unlock panel even when LVDS is disabled

Otherwise we'll have backtraces in assert_panel_unlocked because the
BIOS locks the register. In the reporter's case this regression was
introduced in

commit c31407a3
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Thu Oct 18 21:07:01 2012 +0100

    drm/i915: Add no-lvds quirk for Supermicro X7SPA-H

Reported-by: Alexey Orishko <alexey.orishko@gmail.com>
Cc: Alexey Orishko <alexey.orishko@gmail.com>
Cc: stable@vger.kernel.org
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Francois Tigeot <ftigeot@wolfpond.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Tested-by: Alexey Orishko <alexey.orishko@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>