drm/i915/bios: use ddc pin directly from child data

Avoid extra caching of the data. This is slightly more subtle than one
would think. For one thing, we explicitly ignore 0 value in child device
ddc pin; this is specified as N/A and does not warrant a warning. For
another, we start looking for ddc pin collisions in sanitize using
unmapped pin numbering.

v2: Check !devdata in intel_bios_alternate_ddc_pin()

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/e1dbf7cbdd2191439e760ab9098242dcec5fbb2e.1630512523.git.jani.nikula@intel.com