irqchip/gic-v3-its: Only emit VSYNC if targetting a valid collection

Similarily to the SYNC operation, it must be verified that the VPE
targetted by a VLPI is backed by a valid collection in the GIC driver data
structures.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Yang Yingliang <yangyingliang@huawei.com>
Cc: Sumit Garg <sumit.garg@linaro.org>
Link: https://lkml.kernel.org/r/20180622095254.5906-7-marc.zyngier@arm.com