gpu: host1x: Free the IOMMU domain when there is no device to attach

When there is no device to attach to the IOMMU domain, as may be the
case when the device-tree does not contain the proper iommu node, it is
best to keep going without IOMMU support rather than failing.
This allows the driver to probe and function instead of taking down
all of the tegra drm driver, leading to missing display support.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Fixes: 404bfb78

 ("gpu: host1x: Add IOMMU support")
Reviewed-by: Mikko Perttunen <mperttunen@nvidia.com>
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170710193305.5987-1-contact@paulk.fr