drm/xe: Assert runnable state in handle_sched_done

Ensure G2H and KMD GuC machine match.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240611144053.2805091-8-matthew.brost@intel.com