drm/i915/lnl+/tc: Use the cached max lane count value

Use the cached max lane count value on LNL+, to account for scenarios
where this value is queried after the HW cleared the corresponding pin
assignment value in the TCSS_DDI_STATUS register after the sink got
disconnected.

For consistency, follow-up changes will use the cached max lane count
value on other platforms as well and will also cache the pin assignment
value in a similar way.

Cc: stable@vger.kernel.org # v6.8+
Reported-by: Charlton Lin <charlton.lin@intel.com>
Tested-by: Khaled Almahallawy <khaled.almahallawy@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://lore.kernel.org/r/20250811080152.906216-5-imre.deak@intel.com