If a client is already attached to an IOMMU domain that is not the
shared domain, don't try to attach it again. This allows using the
IOMMU-backed DMA API.

Since the IOMMU-backed DMA API is now supported and there's no way
to detach from it on 64-bit ARM, don't bother to detach from it on
32-bit ARM either.
Signed-off-by: Thierry Reding <treding@nvidia.com>