drm/nouveau/instmem/gk20a: make use of the IOMMU bit

Use the IOMMU bit specified in platform data instead of hardcoding it to
the bit used by current Tegra GPUs.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>