x86: Speed up the irq_remapped check in hot pathes

irq_2_iommu is in struct irq_cfg, so we can do the irq_remapped check
based on irq_cfg instead of going through a lookup function. That's
especially interesting in the eoi_ioapic_irq() hotpath.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>