drm/i915: Eliminate the unused dev_priv->vbt.lvds_vbt

dev_priv->vbt.lvds_vbt is set but never actually used. Kill it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180508124136.10969-3-ville.syrjala@linux.intel.com