drm/i915/dg2: Drop pre-production display workarounds

All production DG2 cards have display stepping C0 or later.  We can drop
Wa_14013215631 (only applies to pre-C0) and make Wa_14010547955
unconditional (applies to everything B0 and beyond).  Also drop the
now-unused IS_DG2_DISPLAY_STEP macro.

Bspec: 44477
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Matt Atwood <matthew.s.atwood@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230816214201.534095-9-matthew.d.roper@intel.com