drm/nouveau/tegra: Stop using iommu_present()

Even if some IOMMU has registered itself on the platform "bus", that
doesn't necessarily mean it provides translation for the device we
care about. Replace iommu_present() with a more appropriate check.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
[added cc for stable]
Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: stable@vger.kernel.org # v5.0+
Link: https://patchwork.freedesktop.org/patch/msgid/70d40ea441da3663c2824d54102b471e9a621f8a.1649168494.git.robin.murphy@arm.com