intel-iommu: set compatibility format interrupt

When extended interrupt mode (x2apic mode) is not supported in a
system, it must set compatibility format interrupt to bypass
interrupt remapping, otherwise compatibility format interrupts
will be blocked.

This will be used when interrupt remapping is enabled while x2apic
is not supported.

Signed-off-by: Weidong Han <weidong.han@intel.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>