drm/i915/bios: use max tmds clock 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/451082780a8befb2b27919439c901a2159ce173f.1630512523.git.jani.nikula@intel.com