CPU pipe B FIFO underrun

I noticed that sometimes when I Alt+tab between my browser and another app, my external display blacks out for about 2 seconds. When this happens, I see this in my dmesg:

```
[drm:intel_cpu_fifo_underrun_irq_handler [i915]] *ERROR* CPU pipe B FIFO underrun
```

I'm not sure whether or not this is an issue with the kernel.
