irqchip/gic-v3-its: Only send VINVALL to a single ITS

Sending VINVALL to all ITSs is completely pointless, as all
we're trying to achieve is to tell the redistributor that
the property table for this VPE should be invalidated.

Let's issue the command on the first valid ITS and be done with it.

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