drm/xe/step: switch to the shared step definitions with i915

drm/xe/step: switch to the shared step definitions with i915

Use the shared stepping enums from include/drm/intel/step.h.

For now, define xe_step as intel_step to avoid mass renames at the same
time. For compat, we can remove the reverse macro.

Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://patch.msgid.link/8173d9d753b343ba127d86277344248a6b1d0c3f.1774875688.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>