drm/i915/bios: create fake child devices on missing VBT

Instead of initialing data directly in ddi_port_info array, create fake
child devices for default outputs when the VBT is missing. This makes
further unification of output handling easier.

This will make intel_bios_is_port_present() return true for the fake
child devices. This may cause subtle changes in a handful of places.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/91675b40a78bd04bf138598d979661257181880d.1615998927.git.jani.nikula@intel.com