drm: Do not force 1024x768 modes on unknown connectors

Only fallback to a set of default modes on a connector iff that
connector is known to be connected. The issue occurs that with limited
hardware which cannot probe a connector and so reports the
connector status as unknown will then attempt to retrieve the modes for
it during drm_helper_probe_single_connector_modes(). Should that fail,
the helper then generates a default set which fools the fb_helper and
causes havoc with the console and beyond.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>