drm/i915/tc: Handle pin assignment NONE on all platforms

drm/i915/tc: Handle pin assignment NONE on all platforms

For consistency, handle pin assignment NONE on all platforms similarly
to LNL+. On earlier platforms the driver doesn't actually see this pin
assignment - as it's not valid on a connected DP-alt PHY - however it's
a valid HW setting even on those platforms, for instance in legacy mode.
Handle this pin assignment on earlier platforms as well, so that the way
to query the pin assignment can be unified by a follow-up change.

Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: https://lore.kernel.org/r/20250805073700.642107-12-imre.deak@intel.com
Signed-off-by: Imre Deak <imre.deak@intel.com>