drm/i915/tc: Fix TC mode for a legacy port if the PHY is not ready

A legacy TC port can't be switched to TBT mode, even if the PHY
initialization wasn't ready yet for some reason, so prevent this.

This shouldn't normally happen as the driver waits for the IOM/TCSS
PHY initialization during driver loading and system resume.

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/20230316131724.359612-9-imre.deak@intel.com