drm/i915: rip out an unused lvds_reg variable

Somehow this has been forgotten in

commit 1974cad0
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Mon Nov 26 17:22:09 2012 +0100

    drm/i915: move is_dual_link_lvds to intel_lvds.c

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>