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 6b9fb7082409 "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>

arch/x86/kernel/acpi/boot.c

@@ -604,14 +604,18 @@ void __init acpi_pic_sci_set_trigger(unsigned int irq, u16 trigger)
 
 int acpi_gsi_to_irq(u32 gsi, unsigned int *irqp)
 {
-	int irq = mp_map_gsi_to_irq(gsi, IOAPIC_MAP_ALLOC | IOAPIC_MAP_CHECK);
+	int irq;
 
-	if (irq >= 0) {
+	if (acpi_irq_model == ACPI_IRQ_MODEL_PIC) {
+		*irqp = gsi;
+	} else {
+		irq = mp_map_gsi_to_irq(gsi,
+					IOAPIC_MAP_ALLOC | IOAPIC_MAP_CHECK);
+		if (irq < 0)
+			return -1;
 		*irqp = irq;
-		return 0;
 	}
-
-	return -1;
+	return 0;
 }
 EXPORT_SYMBOL_GPL(acpi_gsi_to_irq);