drm/i915/bios: iterate over child devices to initialize ddi_port_info

Iterate over child devices instead of ports in parse_ddi_ports() to
initialize ddi_port_info. We'll eventually need to decide some stuff
based on the child device order, which may be different from the port
order.

As a bonus, this allows better abstractions for e.g. dvo port mapping.

There's a subtle change in the DDC pin and AUX channel sanitization as
we change the order. Otherwise, this should not change behaviour.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190322121008.4456-1-jani.nikula@intel.com