drm/amdgpu: fix possible fence leaks from job structure

If we don't end up initializing the fences, free them when
we free the job.  We can't set the hw_fence to NULL after
emitting it because we need it in the cleanup path for the
submit direct case.

v2: take a reference to the fences if we emit them
v3: handle non-job fence in error paths

Fixes: db36632ea5 ("drm/amdgpu: clean up and unify hw fence handling")
Reviewed-by: Jesse Zhang <Jesse.Zhang@amd.com> (v1)
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c

@@ -849,6 +849,8 @@ int amdgpu_vm_flush(struct amdgpu_ring *ring, struct amdgpu_job *job,
 		if (r)
 			return r;
 		fence = &job->hw_vm_fence->base;
+		/* get a ref for the job */
+		dma_fence_get(fence);
 	}
 
 	if (vm_flush_needed) {