drm/xe: Add pending disable assert to handle_sched_done

Will help catch bugs in GuC state machine.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240611144053.2805091-10-matthew.brost@intel.com