iommu/amd: Take into account that alloc_dev_data() may return NULL

find_dev_data() does not check whether the return value alloc_dev_data()
is NULL. This was okay once because the pointer was returned once as-is.
Since commit df3f7a6e ("iommu/amd: Use is_attach_deferred
call-back") the pointer may be used within find_dev_data() so a NULL
check is required.

Cc: Baoquan He <bhe@redhat.com>
Fixes: df3f7a6e ("iommu/amd: Use is_attach_deferred call-back")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>