drm/i915/dg2: Introduce DG2_D subplatform

drm/i915/dg2: Introduce DG2_D subplatform

Introduce DG2_D subplatform for the devices that span across multiple
DG2 subplatforms but are within same segment and will be useful for
segment specific features.

v3: Rework subplatform naming (Jani)
    Split subplatform check into separate case (Jani)

Signed-off-by: Raag Jadav <raag.jadav@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241211115952.1659287-3-raag.jadav@intel.com