drm/i915/dg2: Return Wa_22012654132 to just specific steppings

Programming of the ENABLE_PREFETCH_INTO_IC bit originally showed up in
both the general DG2 tuning guide (applicable to all DG2
variants/steppings) and under Wa_22012654132 (applicable only to
specific steppings).  It has now been removed from the tuning guide, and
the guidance is to only program it in the specific steppings associated
with the workaround.

Bspec: 68331
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Atwood <matthew.s.atwood@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221213234119.2963317-1-matthew.d.roper@intel.com