sched: dynamic: Fix preemption model strings

We recently removed the enum values 'preempt_dynamic_none' and
'preempt_dynamic_voluntary', but forgot to update preempt_modes[]
accordingly. Due to this, preempt_model_str() and sched_dynamic_show()
produce incorrect output, with the wrong preemption model string in
backtraces, and empty output for /sys/kernel/debug/sched/preempt.

Update preempt_modes[] to account for the removed enum values. Given
that CONFIG_PREEMPT_DYNAMIC now depends on CONFIG_ARCH_HAS_PREEMPT_LAZY,
and given that only 'preempt_dynamic_full' and 'preempt_dynamic_lazy'
remain, sched_dynamic_show() no longer needs to skip any model strings.

Fixes: 9650ce11f2e3 ("sched: dynamic: Simplify preempt model accessors")
Reported-by: Jinjie Ruan <ruanjinjie@huawei.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260902101637.232129-1-mark.rutland@arm.com