drm/i915/dp_mst: Use intel_display::platform.alderlake_p instead of IS_ALDERLAKE_P()

Use the driver's standard intel_display::platform.alderlake_p instead of
IS_ALDERLAKE_P().

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250108151916.491113-6-imre.deak@intel.com