drm/i915: Ironlake shares the same video sprite controls as Sandybridge

Well, almost. Just a couple of differences, Ironlake lacks a few of the
RGB formats, only exposing x8r8g8b8, and lacks a couple of unused
features. Given the similarities, we can then reuse the same routines as
already written for Sandybridge to enable overlay support for Ironlake as
well.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>