drm/i915: rename disp_stepping->display_step and gt_stepping->gt_step

Matter of taste. Step matches the enums.

Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1724c8bde0e0f596240437d72ace60b9c34ae9db.1616764798.git.jani.nikula@intel.com