iommu/amd: Don't use dev_data in irte_ga_set_affinity()

search_dev_data() acquires a non-raw lock, which can't be done
from atomic context on PREEMPT_RT.  There is no need to look at
dev_data because guest_mode should never be set if use_vapic is
not set.

Signed-off-by: Scott Wood <swood@redhat.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>