drm/i915: remove crtc disabling special case

Originally this has been introduced in

commit 6eebd6bb
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Mon Nov 28 21:10:05 2011 +0000

    drm: Fix lack of CRTC disable for drm_crtc_helper_set_config(.fb=NULL)

With the improvements of the output state staging and no longer
overwriting crtc->fb before the hw state is updated we can now handle
crtc disabling as part of the normal modeset sequence.

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