drm/kms/fb: provide a 1024x768 fbcon if no outputs found.

If we get no outputs setup provide a 1024x768 fbcon, with
this + radeon hotplug stuff I can plug a monitor in after startup
and get to see stuff.

Last thing is to add some sort of timer for non-hpd outputs like
VGA etc.

Signed-off-by: Dave Airlie <airlied@redhat.com>