drm/i915: split out stepping info to a new file

gt/intel_workarounds.c is decidedly the wrong place for handling
stepping info. Add new intel_step.[ch] for the data, and move the
stepping arrays there. No functional changes.

v2: Rename stepping->step

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/f69baf82819a8a35815fca25a520de5c38a7e1b5.1616764798.git.jani.nikula@intel.com