drm/i915/bios: make child device order the priority order

Make the child device order the priority order in sanitizing DDC pin and
AUX CH. First come, first served.

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/34ab98880386a095422521ad39f4c080eeb3989a.1559308269.git.jani.nikula@intel.com