gpu: host1x: Skip IOMMU initialization if firewall is enabled

Host1x's CDMA can't access the command buffers if IOMMU and Host1x
firewall are enabled in the kernels config because firewall doesn't map
the copied buffer into IOVA space. Fix this by skipping IOMMU
initialization if firewall is enabled as firewall merges sparse cmdbufs
into a single contiguous buffer and hence IOMMU isn't needed in this case.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>