irqchip/gic-v3-its: Move allocation outside mutex

There's no reason to do the allocation of the new lpi_range inside the
lpi_range_lock. One could change the code to avoid the allocation
altogether in case the freed range can be merged with one or two
existing ranges (in which case the allocation would naturally be done
under the lock), but it's probably not worth complicating the code for
that.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>