x86: ACPI: Do not translate GSI number if IOAPIC is disabled

When IOAPIC is disabled, acpi_gsi_to_irq() should return gsi directly
instead of calling mp_map_gsi_to_irq() to translate gsi to IRQ by IOAPIC.
It fixes https://bugzilla.kernel.org/show_bug.cgi?id=84381

.

This regression was introduced with commit 6b9fb708 "x86, ACPI,
irq: Consolidate algorithm of mapping (ioapic, pin) to IRQ number"

Reported-and-Tested-by: Thomas Richter <thor@math.tu-berlin.de>
Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Thomas Richter <thor@math.tu-berlin.de>
Cc: rui.zhang@intel.com
Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: <stable@vger.kernel.org> # 3.17
Link: http://lkml.kernel.org/r/1413816327-12850-1-git-send-email-jiang.liu@linux.intel.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>