drm/i915: Make the force_thru workaround atomic, v2.

Set connectors_changed to force a modeset if the panel fitter's force
enabled on eDP.

Changes since v1:
- Use connectors_changed instead of active_changed because it's a
  routing update.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>