drm/i915: Only populate aux_ch if really needed

Mixing VBT based AUX CH with platform defaults seems like
a recipe for conflicts. Let's only populate AUX CH if we
absolutely need it, that is only if we are dealing with
a DP output or a TC port (which need it due to some power
well shenanigans).

TODO: double check that real VBTs do in fact populate
      the AUX CH for HDMI TC legacy ports...

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230630155846.29931-3-ville.syrjala@linux.intel.com