irqchip/gicv3-its: Skip irq affinity setting when target cpu is the same as current setting

Just skip the irq affinity setting when the target cpu is the same as
current setting.
This is a small optimization for irq affinity setting logic.

Signed-off-by: MaJun <majun258@huawei.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>