drm/nouveau/platform: allow to specify the IOMMU bit

Current Tegra code taking advantage of the IOMMU assumes a hardcoded
value for the IOMMU bit. Make it a platform property instead for
flexibility.

v2 (Ben Skeggs): remove nvkm dependence on drm structures

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