drm/i915/bios: use hdmi level shift directly from child data

Avoid extra caching of the data.

Cc: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ef22e40b01eab571ff0dc2bfffabb906d0151fb4.1630512523.git.jani.nikula@intel.com