x86, ioapic: Optimize pin_2_irq

Now that all ioapics have valid gsi_base values use this to
accellerate pin_2_irq.  In the case of acpi this also ensures
that pin_2_irq will compute the same irq value for an ioapic
pin as acpi will.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
LKML-Reference: <1269936436-7039-12-git-send-email-ebiederm@xmission.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>