drm: Don't swallow error codes in drm_dev_alloc()

There are many reasons other than ENOMEM that drm_dev_init() can
fail. Return ERR_PTR rather than NULL to be able to distinguish
these in the caller.

Signed-off-by: Tom Gundersen <teg@jklm.no>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20160921145919.13754-2-teg@jklm.no

drivers/gpu/drm/nouveau/nouveau_drm.c

@@ -1067,8 +1067,8 @@ nouveau_platform_device_create(const struct nvkm_device_tegra_func *func,
 		goto err_free;
 
 	drm = drm_dev_alloc(&driver_platform, &pdev->dev);
-	if (!drm) {
-		err = -ENOMEM;
+	if (IS_ERR(drm)) {
+		err = PTR_ERR(drm);
 		goto err_free;
 	}