drm/i915: use i915 and Ironlake CRTC enable/disable functions in prepare/commit

This will allow us to optimize our prepare/commit paths a bit better.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[ickle: minor tweak to handle the cursor across pipe resizing]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>