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: 45c690aea8ee ("drm/tegra: Use iommu_paging_domain_alloc()")
Signed-off-by: Dan Carpenter <[e-mail]>
Reviewed-by: Lu Baolu <[e-mail]>
Signed-off-by: Thierry Reding <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/tegra/drm.c

···
 
 	if (host1x_drm_wants_iommu(dev) && device_iommu_mapped(dma_dev)) {
 		tegra->domain = iommu_paging_domain_alloc(dma_dev);
-		if (!tegra->domain) {
-			err = -ENOMEM;
+		if (IS_ERR(tegra->domain)) {
+			err = PTR_ERR(tegra->domain);
 			goto free;
 		}