drm/i915: 830M doesn't have an LVDS port

There's no LVDS port on 830M so don't go reading the LVDS control
register.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>