etnaviv: fix submit error path

If the gpu submit fails, bail out to avoid accessing a potentially
unititalized fence.

CC: stable@vger.kernel.org #4.12+
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>

drivers/gpu/drm/etnaviv/etnaviv_gem_submit.c

@@ -445,8 +445,10 @@ int etnaviv_ioctl_gem_submit(struct drm_device *dev, void *data,
 	cmdbuf->user_size = ALIGN(args->stream_size, 8);
 
 	ret = etnaviv_gpu_submit(gpu, submit, cmdbuf);
-	if (ret == 0)
-		cmdbuf = NULL;
+	if (ret)
+		goto out;
+
+	cmdbuf = NULL;
 
 	if (args->flags & ETNA_SUBMIT_FENCE_FD_OUT) {
 		/*