drm/i915: Inject a failure point when registering a connector

Check we can handle a late display load failure where the final act of
registering the connector fails.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Jani Nikula <jani.nikula@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181010123833.16797-1-chris@chris-wilson.co.uk