drm/i915/display: Add MTL subplatforms definition

Separate MTL-U platform PCI ids in one define macro.

Add the MTL U/ARL U as subplatform member in MTL platform description
structure to use display.platform.<platform> from intel_display
structure instead of IS_<PLATFORM>() in display code path.

v2:
- Club ARL-u in MTL and identify ARL-u as MTL-u subplatform(Jani)

Signed-off-by: Dnyaneshwar Bhadane <dnyaneshwar.bhadane@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241217201301.3593054-2-dnyaneshwar.bhadane@intel.com