drm/msm: Fix a fence leak in submit error path

In the Linux kernel, the following vulnerability has been resolved:

drm/msm: Fix a fence leak in submit error path

In error paths, we could unref the submit without calling drm_sched_entity_push_job(), so msm_job_free() will never get called. Since drm_sched_job_cleanup() will NULL out the s_fence, we can use that to detect this case.

Patchwork: https://patchwork.freedesktop.org/patch/653584/