iommu/vt-d: Tylersburg isoch identity map check is done too late.

The check to set identity map for tylersburg is done too late. It needs
to be done before the check for identity_map domain is done.

To: Joerg Roedel <joro@8bytes.org>
To: David Woodhouse <dwmw2@infradead.org>
Cc: iommu@lists.linux-foundation.org
Cc: linux-kernel@vger.kernel.org
Cc: stable@vger.kernel.org
Cc: Ashok Raj <ashok.raj@intel.com>

Fixes: 86080ccc ("iommu/vt-d: Allocate si_domain in init_dmars()")
Signed-off-by: Ashok Raj <ashok.raj@intel.com>
Reported-by: Yunhong Jiang <yunhong.jiang@intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>