drm/i915: Only skip connector output for disable_display

We want to add no connectors, encoders or crtcs if the display is
disabled, but we still need to hook up any existing HW so that we can
power it down.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180815201207.2203-1-chris@chris-wilson.co.uk