sched_ext: Fix processs_ddsp_deferred_locals() by unifying DTL_INVALID handling

With the preceding update, the only return value which makes meaningful
difference is DTL_INVALID, for which one caller, finish_dispatch(), falls
back to the global DSQ and the other, process_ddsp_deferred_locals(),
doesn't do anything.

It should always fallback to the global DSQ. Move the global DSQ fallback
into dispatch_to_local_dsq() and remove the return value.

v2: Patch title and description updated to reflect the behavior fix for
    process_ddsp_deferred_locals().

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: David Vernet <void@manifault.com>