drm/xe/pvc: Remove A* steppings

The PVC pre-production A* steppings are not going to be supported in xe
driver - the steppings are important for the WAs and since we are not
adding the pre-productions ones, there is no need to add the stepping.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>