iommu/vt-d: Avoid duplicated pci dma alias consideration

As we have abandoned the home-made lazy domain allocation
and delegated the DMA domain life cycle up to the default
domain mechanism defined in the generic iommu layer, we
needn't consider pci alias anymore when mapping/unmapping
the context entries. Without this fix, we see kernel NULL
pointer dereference during pci device hot-plug test.

Cc: Ashok Raj <ashok.raj@intel.com>
Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: Kevin Tian <kevin.tian@intel.com>
Fixes: fa954e68 ("iommu/vt-d: Delegate the dma domain to upper layer")
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reported-and-tested-by: Xu Pengfei <pengfei.xu@intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>