drm/i915: Enable aspect/centering panel fitting for Ironlake.

v2: Hook in DP paths to keep FULLSCREEN panel fitting on eDP.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>