drm/i915: Move aux_ch to intel_digital_port

From ICL onwards all DDI/TypeC ports - even working in HDMI mode - need
to know their corresponding AUX CH, so move the field to a common
struct.

No functional change.

v3:
- Add code comment about which ports aux_ch is used for. (Jose)

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181101140427.31026-3-imre.deak@intel.com