drm/xe: Group engine related structs

Move the definition of drm_xe_engine_class_instance to group it with
other engine related structs and to follow the ioctls order.

Reported-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Francois Dugast <francois.dugast@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>