sched, sched_ext: Disable SM_IDLE/rq empty path when scx_enabled()

Disable the rq empty path when scx is enabled. SCX must consult the BPF
scheduler (via the dispatch path in balance) to determine if rq is empty.

This fixes stalls when scx is enabled.

Signed-off-by: Pat Somaru <patso@likewhatevs.io>
Fixes: 3dcac251 ("sched/core: Introduce SM_IDLE and an idle re-entry fast-path in __schedule()")
Signed-off-by: Tejun Heo <tj@kernel.org>