drm/i915: rename i915_rev_steppings->intel_step_info

Matter of taste. Match the prefix for everything else related to
steppings. No functional changes.

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/d3ba7c448e596fd2425a29bcc7ea8493505a30f9.1616764798.git.jani.nikula@intel.com