drm/i915/display: identify display steppings in display probe

Both i915 and xe have code to identify display steppings. Start
deduplicating this by, uh, adding a third copy in display code. This is
not yet used for anything other than debug logging. We'll switch over
later.

For platforms before GMD ID, attach the mapping from PCI revision to
stepping in the platform and subplatform descriptors. This is a
considerably cleaner approach than having it completely separate.

Also add a separate field for stepping in display runtime info,
preserving the value from GMD ID.

v2: Handle NULL subdesc (Matt)

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Acked-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240821095036.2044654-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

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