x86/ioapic: use legacy_pic to set correct gsi-irq mapping

x86/ioapic: use legacy_pic to set correct gsi-irq mapping

Using compile time NR_LEGACY_IRQS causes the wrong gsi-irq mapping on
non-PC platforms, such as Moorestown. This patch uses legacy_pic abstraction
to set the correct number of legacy interrupts at runtime. For Moorestown,
nr_legacy_irqs = 0. We have 1:1 mapping for gsi-irq even within the legacy
irq range.

Change-Id: I3ab904292f1270677a55a08a4c1134681a2efd50
Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Dirk Brandewie <dirk.brandewie@gmail.com>