drm/i915/dg1: Drop support for pre-production steppings

Several post-DG1 platforms have been brought up now, so we're well past
the point where we usually drop the workarounds that are only applicable
to internal/pre-production hardware.

Production DG1 hardware always has a B0 stepping for both display and
GT.

Bspec: 44463
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230127224313.4042331-3-matthew.d.roper@intel.com