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

Several post-TGL 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 TGL hardware always has display stepping C0 or later and GT
stepping B0 or later (this is true for both the original TGL and the U/Y
subplatform).

Bspec 44455
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/20230127224313.4042331-2-matthew.d.roper@intel.com