Skylake underruns on 4.8-rc4

[Andy Lutomirski]

My Dell XPS 13 9350 laptop just got a buffer underrun:

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

I'm seeing this very occasionally, and they don't come in groups -- I
seem to get one underrun with a black flash and that's it.  This is
with just the laptop screen -- nothing at all is plugged in to the
USB-C port.

4.8-rc4 has the latest round of fixes applied, so
i915/skl_dmc_ver1_26.bin loaded successfully and the SAGV fix is
there.

I had the same problem on 4.8-rc3.  4.7 seemed okay.

I have:

00:02.0 VGA compatible controller: Intel Corporation HD Graphics 520 (rev 07)

--Andy