drm/nouveau/tegra: Use iommu_paging_domain_alloc()

In nvkm_device_tegra_probe_iommu(), a paging domain is allocated for @dev
and attached to it on success. Use iommu_paging_domain_alloc() to make it
explicit.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240902014700.66095-2-baolu.lu@linux.intel.com