dmc/i915/tc: Report pin assignment NONE in TBT-alt mode

dmc/i915/tc: Report pin assignment NONE in TBT-alt mode

The pin assignment is only relevant in case the PHY is owned by the
display, that is in legacy and DP-alt mode. In TBT-alt mode the PHY is
owned by the TBT FW/driver and so the pin assignment/configuration is
managed by those components. A follow-up change will cache the pin
assignment value in all the TypeC modes - querying this by calling
get_pin_assignment() - prepare for that here, by reporting pin
assignment NONE in the TBT-alt mode.

Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: https://lore.kernel.org/r/20250805073700.642107-18-imre.deak@intel.com
Signed-off-by: Imre Deak <imre.deak@intel.com>