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: eb3961a5 ("drm/amdgpu: remove fence context from the job")
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>