drm/i915: Clean up LVDS register handling harder

drm/i915: Clean up LVDS register handling harder

Minor fixup to d0669d007542 ("drm/i915: Clean up LVDS register
handling") which intended to read lvds_reg just once at the
beginning of intel_lvds_init() and use that throughout the rest
of the function but accidentally missed one register readout.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20151107141244.AB7616E242@gabe.freedesktop.org
Signed-off-by: Jani Nikula <jani.nikula@intel.com>