drm/i915: Pimp AUX CH names

Let's make the AUX CH names match the spec (AUX A-F for pre-tgl,
AUX A-C or AUX USBC1-6 for tgl+). And while at it let's include
the full encoder name in the AUX CH name as well (as opposed to
just using port_name() which wouldn't give us the right thing on
tgl+).

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/20201028213323.5423-6-ville.syrjala@linux.intel.com