drm/xe: 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/19bc7a3197f2bc6f3c0d337487ab19f3b7f5612a.1724180287.git.jani.nikula@intel.com


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