sched_ext: Make find_dsq_for_dispatch() handle SCX_DSQ_LOCAL_ON

find_dsq_for_dispatch() handles all DSQ IDs except SCX_DSQ_LOCAL_ON.
Instead, each caller is hanlding SCX_DSQ_LOCAL_ON before calling it. Move
SCX_DSQ_LOCAL_ON lookup into find_dsq_for_dispatch() to remove duplicate
code in direct_dispatch() and dispatch_to_local_dsq().

No functional changes intended.

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