iommu/tegra: gart: Do not register with bus

The driver currently doesn't work as expected and causes existing setups
with Tegra20 to break after commit df06b759 ("drm/tegra: Add IOMMU
support"). To restore these setups, do not register the operations with
the platform bus for now. Fixing this properly will involve non-trivial
changes to the DRM driver, which are unlikely to be accepted at this
point in the release cycle.

Reported-by: Misha Komarovskiy <zombah@gmail.com>
Reported-by: Nicolas Chauvet <kwizart@gmail.com>
Tested-by: Misha Komarovskiy <zombah@gmail.com>
Tested-by: Dmitry Osipenko <digetx@gmail.com>
Cc: Hiroshi Doyu <hdoyu@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>