powerpc: Fix preempt-lazy on 64bit

Schedule must not be invoked on a lazy-preempt request if preemption is
disabled. This regression was introduced in v5.9-rc2-rt1.

Check the preemption counter before looking at the lazy-preemption
counter and both have to be 0.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>