drm/i915: Clean up LVDS register handling

Keep single 'lvds_reg' and 'lvds' variable around in
intel_lvds_init(), and read it just once at the start.

Also intel_lvds_get_config() doesn't need to figure out which reg to use
since it can just consult lvds_encoder->reg.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>