drm/tegra: Fix NULL vs IS_ERR() check in probe()

The iommu_paging_domain_alloc() function doesn't  return NULL pointers,
it returns error pointers.  Update the check to match.

Fixes: 45c690ae ("drm/tegra: Use iommu_paging_domain_alloc()")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ba31cf3a-af3d-4ff1-87a8-f05aaf8c780b@stanley.mountain