x2APIC/VT-d: allocate iommu when create a drhd

x2APIC/VT-d: allocate iommu when create a drhd

A drhd is created when parse ACPI DMAR table, but drhd->iommu is not
allocated until iommu setup. But iommu is needed by x2APIC which will
enable interrupt remapping before iommu setup. This patch allocates
iommu when create drhd. And then drhd->ecap can be removed because
it's the same as iommu->ecap.

Signed-off-by: Weidong Han <weidong.han@intel.com>