x86/amd-iommu: Simplify get_device_resources()

With the previous changes the get_device_resources function
can be simplified even more. The only important information
for the callers is the protection domain.
This patch renames the function to get_domain() and let it
only return the protection domain for a device.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>