drm/i915/dg2: Drop Wa_16011777198

Wa_16011777198 only applies to pre-production steppings of DG2, which
we're no longer supporting.  Remove the workaround and override_gucrc
handling, which is no longer needed.  Since this was the final use of
IS_DG2_GRAPHICS_STEP, that macro can also be removed now.

v2:
 - Include the promised removal of override_gucrc handling.

Cc: Ashutosh Dixit <ashutosh.dixit@intel.com>
Cc: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Cc: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230816214824.548575-2-matthew.d.roper@intel.com