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/cf2dccd1c9c7fdcf5de08ea10a9265292b45d8c7.1616764798.git.jani.nikula@intel.com