sched_ext: Move scx_buildin_idle_enabled check to scx_bpf_select_cpu_dfl()

Move the sanity check from the inner function scx_select_cpu_dfl() to the
exported kfunc scx_bpf_select_cpu_dfl(). This doesn't cause behavior
differences and will allow using scx_select_cpu_dfl() in bypass mode
regardless of scx_builtin_idle_enabled.

Signed-off-by: Tejun Heo <tj@kernel.org>