drm/panfrost: Make panfrost_job_run() return an ERR_PTR() instead of NULL

If we don't do that, dma_fence_set_error() complains (called from
drm_sched_main()).

Fixes: f3ba9122 ("drm/panfrost: Add initial panfrost driver")
Cc: <stable@vger.kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>

drivers/gpu/drm/panfrost/panfrost_job.c

@@ -327,13 +327,13 @@ static struct dma_fence *panfrost_job_run(struct drm_sched_job *sched_job)
 	struct dma_fence *fence = NULL;
 
 	if (unlikely(job->base.s_fence->finished.error))
-		return NULL;
+		return ERR_PTR(job->base.s_fence->finished.error);
 
 	pfdev->jobs[slot] = job;
 
 	fence = panfrost_fence_create(pfdev, slot);
 	if (IS_ERR(fence))
-		return NULL;
+		return ERR_PTR(-ENOMEM);
 
 	if (job->done_fence)
 		dma_fence_put(job->done_fence);