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"