drm/i915/tc: Fix target TC mode for a disconnected legacy port

Atm, the target TC mode - which the PHY should be switched to at any
point it's used - is TBT in case there is no sink connected. However
legacy ports are only used in the legacy mode regardless of the sink
connected state. Fix the mode returned by
intel_tc_port_get_target_mode() accordingly.

Despite of the above issue, the PHY got disconnected as expected in
response to a sink disconnect event, causing only a redundant
PHY disconnect->reconnect sequence whenever the port was used.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230316131724.359612-8-imre.deak@intel.com