drm/i915: The sprite scaler on Ironlake also support YUV planes

This fixes a regression from

commit 57779d06
Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
Date:   Wed Oct 31 17:50:14 2012 +0200

    drm/i915: Fix display pixel format handling

(which even says that they are supported on Ironlake, and then promptly
rejects then...)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>