iommu/vt-d: Allow IR works in XAPIC mode though CPU works in X2APIC mode

Currently if CPU supports X2APIC, IR hardware must work in X2APIC mode
or disabled. Change the code to support IR working in XAPIC mode when
CPU supports X2APIC. Then the CPU APIC driver will decide how to handle
such as configuration by:
1) Disabling X2APIC mode
2) Forcing X2APIC physical mode

This change also fixes a live locking when
1) BIOS enables CPU X2APIC
2) DMAR table disables X2APIC mode or IR hardware doesn't support X2APIC
with following messages:
[   37.863463] dmar: INTR-REMAP: Request device [[f0:1f.7] fault index 2
[   37.863463] INTR-REMAP:[fault reason 36] Detected reserved fields in the IRTE entry
[   37.879372] dmar: INTR-REMAP: Request device [[f0:1f.7] fault index 2
[   37.879372] INTR-REMAP:[fault reason 36] Detected reserved fields in the IRTE entry
[   37.895282] dmar: INTR-REMAP: Request device [[f0:1f.7] fault index 2
[   37.895282] INTR-REMAP:[fault reason 36] Detected reserved fields in the IRTE entry
[   37.911192] dmar: INTR-REMAP: Request device [[f0:1f.7] fault index 2

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Tested-by: Joerg Roedel <joro@8bytes.org>
Cc: Tony Luck <tony.luck@intel.com>
Cc: iommu@lists.linux-foundation.org
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Link: http://lkml.kernel.org/r/1420615903-28253-11-git-send-email-jiang.liu@linux.intel.com


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