gpu: host1x: Return error when context device not attached to IOMMU

If a context device was not attached to IOMMU, we kept the old
success err value causing context devices to be unregistered but
success to be returned. This would mean that things would go on
but with context isolation disabled.

To decide on an explicit behavior, let's return an error code
here instead. If someone wants to go without IOMMU on a platform
modern enough to support context isolation, they can remove the
context devices from device tree.

Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230413082202.114721-2-cyndis@kapsi.fi