xen/arm: IRQ: move gic {, un}lock in gic_set_irq_properties

xen/arm: IRQ: move gic {, un}lock in gic_set_irq_properties

The function gic_set_irq_properties is only called in two places:
    - gic_route_irq: the gic.lock is only taken for the call to the
    former function.
    - gic_route_irq_to_guest: the gic.lock is taken for the duration of
    the function. But the lock is only useful when gic_set_irq_properties.

So we can safely move the lock in gic_set_irq_properties and restrict the
critical section for the gic.lock in gic_route_irq_to_guest.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>