iommu: arm-smmu: Don't blindly use first SMR to calculate mask

With the SMRs inherited from the bootloader the first SMR might actually
be valid and in use. As such probing the SMR mask using the first SMR
might break a stream in use. Search for an unused stream and use this to
probe the SMR mask.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Ondrej Kubik <ondrej.kubik@canonical.com>