drm/i915: Don't propagate the gen split confusion further

There's no such thing as gen13. It is either display 13
or graphics 13. Don't propagate the gen12 confusion
further.

Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211015091650.87270-1-rodrigo.vivi@intel.com