iommu/amd: Remove duplicate checking code

amd_iommu_rlookup_table[devid] != NULL is already guaranteed
by check_device called before, it's fine to attach device at
this point.

Signed-off-by: Vaughan Cao <vaughan.cao@oracle.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>