drm/i915: remove display stepping handling

The code is now unused. Remove.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Acked-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/d64c5e8563dcb8858569d5578230f6b675acdafb.1724180287.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>