drm/i915: copy&paste drm_crtc_helper_set_config

And the following static functions required by it:
drm_encoder_crtc_ok, drm_crtc_helper_disable

No changes safe for the s/drm/intel prefix change.

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