drm/xe: Add stepping support for GMD_ID platforms

drm/xe: Add stepping support for GMD_ID platforms

For platforms with GMD_ID registers, the IP stepping should be
determined from the 'revid' field of those registers rather than from
the PCI revid.

The hardware teams have indicated that they plan to keep the revid =>
stepping mapping consistent across all GMD_ID platforms, with major
steppings (A0, B0, C0, etc.) having revids that are multiples of 4, and
minor steppings (A1, A2, A3, etc.) taking the intermediate values.  For
now we'll trust that hardware follows through on this plan; if they have
to change direction in the future (e.g., they wind up needing something
like an "A4" that doesn't fit this scheme), we can add a GMD_ID-based
lookup table when the time comes.

v2:
 - Set xe->info.platform before finding stepping; the pre-GMD_ID code
   relies on this value to pick a lookup table.
v3:
 - Also set xe->info.subplatform before picking the stepping for
   pre-GMD_ID lookup.

Reviewed-by: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com>
Link: https://lore.kernel.org/r/20230524185952.666158-1-matthew.d.roper@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>