drm/radeon/kms: Forbid creation of framebuffer with no valid GEM object

This will avoid oops if at later point the fb is use. Trying to create
a framebuffer with no valid GEM object is bogus and should be forbidden
as this patch does.
Signed-off-by: NJerome Glisse <jglisse@redhat.com>
Signed-off-by: NDave Airlie <airlied@linux.ie>

drivers/gpu/drm/radeon/radeon_display.c

@@ -715,7 +715,11 @@ radeon_user_framebuffer_create(struct drm_device *dev,
 	struct drm_gem_object *obj;
 
 	obj = drm_gem_object_lookup(dev, file_priv, mode_cmd->handle);
-
+	if (obj ==  NULL) {
+		dev_err(&dev->pdev->dev, "No GEM object associated to handle 0x%08X, "
+			"can't create framebuffer\n", mode_cmd->handle);
+		return NULL;
+	}
 	return radeon_framebuffer_create(dev, mode_cmd, obj);
 }