drm/xe/hw_engine_class_sysfs: Allow to inject error during probe

drm/xe/hw_engine_class_sysfs: Allow to inject error during probe

Allow fault injection in a function used during initialization by
xe_hw_engine_class_sysfs_init() so that its error handling can be
tested.

Signed-off-by: Francois Dugast <francois.dugast@intel.com>
Reviewed-by: Tejas Upadhyay <tejas.upadhyay@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250314105050.636983-1-francois.dugast@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>