drm/i915/bios: use dp max link rate 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/5593ce81f6670dc33cf18fa284f8237c875ef404.1630512523.git.jani.nikula@intel.com