drm: Add default modes for connectors in unknown state

The DRM CRTC helpers add default modes to connectors in the connected
state if no mode can be retrieved from the connector. This behaviour is
useful for VGA or DVI outputs that have no connected DDC bus. However,
in such cases, the status of the output usually can't be retrieved and
is reported as connector_status_unknown.

Extend the addition of default modes to connectors in an unknown state
to support outputs that can retrieve neither the modes nor the
connection status.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>