drm/amdgpu: Fix KFD-related kernel oops on Hawaii

Hawaii needs to flush caches explicitly, submitting an IB in a user
VMID from kernel mode. There is no s_fence in this case.

Fixes: eb3961a57424 ("drm/amdgpu: remove fence context from the job")
Signed-off-by: Felix Kuehling <[e-mail]>
Reviewed-by: Christian König <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

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

···
 	/* ring tests don't use a job */
 	if (job) {
 		vm = job->vm;
-		fence_ctx = job->base.s_fence->scheduled.context;
+		fence_ctx = job->base.s_fence ?
+			job->base.s_fence->scheduled.context : 0;
 	} else {
 		vm = NULL;
 		fence_ctx = 0;