x86/amd-iommu: Fix NULL pointer dereference in __detach_device()

In the __detach_device function the reference count for a
device-domain binding may become zero. This results in the
device being removed from the domain and dev_data->domain
will be NULL. This is bad because this pointer is
dereferenced when trying to unlock the domain->lock. This
patch fixes the issue by keeping the domain in a seperate
variable.

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