x86: Unify 3 similar ways of saving mp_irqs info

There are 3 places defining similar functions of saving IRQ vector
info into mp_irqs[] array: mmparse/acpi/mrst.

Replace the redundant code by a common function in io_apic.c as it's
only called when CONFIG_X86_IO_APIC=y

Signed-off-by: Feng Tang <feng.tang@intel.com>
Cc: Alan Cox <alan@linux.intel.com>
Cc: Len Brown <len.brown@intel.com>
Cc: Yinghai Lu <yinghai@kernel.org>
LKML-Reference: <20101207133204.4d913c5a@feng-i7>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>