drm/i915: add pipe_config->limited_color_range

Now that we have a useful struct for this, let's use it. Some neat
pointer-chasing required, but it's all there already.

v2: Rebased on top of the added Haswell limited color range support.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>