drm/i915/bios: store child device pointer in DDI port info

This allows us to avoid iterating the child devices in some cases.

Also replace the presence bit with child device being non-NULL, and set
the child device pointer last to allow us to take advantage of it in
follow-up work.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
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/ceccb75d637af3134d0328d67cbd6623932f94db.1559308269.git.jani.nikula@intel.com