drm/i915: Only populate aux_ch if really needed

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 <[email protected]>
Signed-off-by: Ville Syrjälä <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]