iommu: Check dev->iommu in dev_iommu_priv_get() before dereferencing it

The dev_iommu_priv_get() needs a similar check to
dev_iommu_fwspec_get() to make sure no NULL-ptr is dereferenced.

Fixes: 05a0542b ("iommu/amd: Store dev_data as device iommu private data")
Cc: stable@vger.kernel.org	# v5.8+
Link: https://lore.kernel.org/r/20210202145419.29143-1-joro@8bytes.org
Reference: https://bugzilla.kernel.org/show_bug.cgi?id=211241


Signed-off-by: Joerg Roedel <jroedel@suse.de>