sched_ext: Restructure dispatch_to_local_dsq()

Now that there's nothing left after the big if block, flip the if condition
and unindent the body.

No functional changes intended.

v2: Add BUG() to clarify control can't reach the end of
    dispatch_to_local_dsq() in UP kernels per David.

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