drm/i915/dg2: s/DISP_STEPPING/DISPLAY_STEPPING/

Commit cd0fcf5a ("drm/i915: rename DISP_STEPPING->DISPLAY_STEP and
GT_STEPPING->GT_STEP") renamed all platforms' display stepping tests,
but the DG2 patches were still in-flight at that time and did not
incorporate the new naming scheme.  Rename DG2's macro now for
consistency with other platforms.

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211116174818.2128062-2-matthew.d.roper@intel.com