drm/i915/bios: move aux ch and ddc checks at a lower level

Unify the code paths at the higher level.

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/44559ef456015f65a863c3d89a9bea9157d13a05.1615998927.git.jani.nikula@intel.com