drm/xe: Promote xe_hw_engine_class_to_str()

Move it out of the sysfs compilation unit so it can be re-used in other
places.

Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Oak Zeng <oak.zeng@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240517204310.88854-2-lucas.demarchi@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>