drm/i915: Use AUX_CH_USBCn for the RKL VBT AUX CH setup

As with the VBT DVO port, RKL uses PHY based mapping for the
VBT AUX CH. Adjust the code to use the new AUX_USBCn names
and add a comment to explain the situation.

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201028213323.5423-7-ville.syrjala@linux.intel.com