irqchip: gicv3-its: Move some alloc/free code to activate/deactivate

The ITS code could do a bit less in the alloc/free paths, and a bit
more in the activate/deactivate methods, giving a better separation
between software allocation and HW programing.

Suggested-by: Wuyun Wu (Abel) <wuyun.wu@huawei.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: Yun Wu (Abel) <wuyun.wu@huawei.com>
Cc: Robert Richter <robert.richter@caviumnetworks.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>