Add sanity check in iris_vb2_stop_streaming. If inst->state is
already IRIS_INST_ERROR, we should skip the stream_off operation
because it would still send packets to the firmware.
In iris_kill_session, inst->state is set to IRIS_INST_ERROR and
session_close is executed, which will kfree(inst_hfi_gen2->packet).
If stop_streaming is called afterward, it will cause a crash.