drm/i915: fix indentation on skl stepping info

Reviewed-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1445344713-1407-1-git-send-email-jani.nikula@intel.com