irqchip/gic-v3: Always trust the managed affinity provided by the core code

Now that the core code has been fixed to always give us an affinity
that only includes online CPUs, directly use this affinity when
computing a target CPU.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20220405185040.206297-4-maz@kernel.org