drm/i915: Initialize dig_port->aux_ch to NONE to be sure

Make sure dig_port->aux_ch is trustworthy by initializing it
to NONE (-1) at the start. The encoder init will later fill in
the actual value, if appropriate.

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/20230630155846.29931-2-ville.syrjala@linux.intel.com