On Tegra234, engines that are programmed through Host1x channels can
be attached to either the NISO0 or NISO1 SMMU. Because of that, when
selecting a context device to use with an engine, we need to select
one that is also attached to the same SMMU.

Add a parameter to host1x_memory_context_alloc to specify which device
we are allocating a context for, and use it to pick an appropriate
context device.
Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
[treding@nvidia.com: update !IOMMU_API stub signature]
Signed-off-by: Thierry Reding <treding@nvidia.com>