drm/i915: prepare load-detect pipe code for dpms changes

A few things need adjustement:
- Change the dpms state by calling the dpms connector function and
  not some crtc helper internal callbacks. Otherwise this will break
  once we switch to our own dpms handling.
- Instead of tracking and restoring intel_crtc->dpms_mode use the
  connector's dpms variable - the former relies on the dpms compuation
  rules used by the crtc helper. And it would break when the encoder
  is cloned and the other output has a different dpms state. But luckily
  no one is crazy enough for that.
- Properly clear the connector -> encoder -> crtc linking, even when
  failing (note that the crtc helper removes the encoder -> crtc link
  in disabled_unused_functions for us).

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