Overexposed IOP: disable piece unless it is really needed

This fixes longstanding FIXME, and reduces time DT_DEV_PIXELPIPE_FULL of by: (overexposed was not enabled)
[dev_pixelpipe] took 0.005 secs (0.004 CPU) processing `overexposed' on GPU, blending on GPU [full]
[dev_pixelpipe] took 0.004 secs (0.008 CPU) processing `overexposed' on CPU, blending on CPU [full]