drm/nouveau/platform: fix compile error if !CONFIG_IOMMU

The lack of IOMMU API support can make nouveau_platform_probe_iommu()
fail to compile because struct iommu_ops is then empty. Fix this by
skipping IOMMU probe in that case - lack of IOMMU on platform devices
is sub-optimal, but is not an error.

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