genirq: Use irq_resolve_mapping() to implement __handle_domain_irq() and co

In order to start reaping the benefits of irq_resolve_mapping(),
start using it in __handle_domain_irq() and handle_domain_nmi().

This involves splitting generic_handle_irq() to be able to directly
provide the irq_desc.

Signed-off-by: Marc Zyngier <maz@kernel.org>