drm/i915/bios: use alternate aux channel directly from child data

Avoid extra caching of the data.

v2: Check for !info->devdata in intel_bios_port_aux_ch() (Ankit)

Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Ankit Nautiyal <ankit.k.nautiyal@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/b1b9f0032b353c3279b4546d6acdb696fe0b6136.1630512523.git.jani.nikula@intel.com