iommu, x86: Restructure setup of the irq remapping feature

iommu, x86: Restructure setup of the irq remapping feature

enable_IR_x2apic() calls setup_irq_remapping_ops() which by default
installs the intel dmar remapping ops and then calls the amd iommu irq
remapping prepare callback to figure out whether we are running on an
AMD machine with irq remapping hardware.

Right after that it calls irq_remapping_prepare() which pointlessly
checks:
	if (!remap_ops || !remap_ops->prepare)
               return -ENODEV;
and then calls

    remap_ops->prepare()

which is silly in the AMD case as it got called from
setup_irq_remapping_ops() already a few microseconds ago.

Simplify this and just collapse everything into
irq_remapping_prepare().

The irq_remapping_prepare() remains still silly as it assigns blindly
the intel ops, but that's not scope of this patch.

The scope here is to move the preperatory work, i.e. memory
allocations out of the atomic section which is required to enable irq
remapping.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Borislav Petkov <bp@alien8.de>
Acked-and-tested-by: Joerg Roedel <joro@8bytes.org>
Cc: Tony Luck <tony.luck@intel.com>
Cc: iommu@lists.linux-foundation.org
Cc: Joerg Roedel <jroedel@suse.de>
Cc: H. Peter Anvin <hpa@linux.intel.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: HATAYAMA Daisuke <d.hatayama@jp.fujitsu.com>
Cc: Jan Beulich <JBeulich@suse.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Oren Twaig <oren@scalemp.com>
Cc: x86@kernel.org
Link: http://lkml.kernel.org/r/20141205084147.232633738@linutronix.de
Link: http://lkml.kernel.org/r/1420615903-28253-2-git-send-email-jiang.liu@linux.intel.com
Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>