x86/amd-iommu: Let domain_for_device handle aliases

If there is no domain associated to a device yet and the
device has an alias device which already has a domain, the
original device needs to have the same domain as the alias
device.
This patch changes domain_for_device to handle this
situation and directly assigns the alias device domain to
the device in this situation.
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>