[Intel-gfx] SNA doesn't work if external monitor is attached during boot

[Grant]

>> SNA works great for me and drastically improves my video playback
>> performance.  However, performance reverts back to non-SNA levels
>> if an external monitor is attached during boot.  SNA works fine if
>> the external monitor is attached after the system has already
>> booted.  I'm on a Dell XPS 13 Ultrabook on xf86-video-intel-2.20.6
>> in Gentoo.
>
> same bug?
> https://bugs.freedesktop.org/show_bug.cgi?id=48652

I believe that bug is about losing the external monitor connection
entirely. My issue seems to be SNA working or not.  SNA is mentioned
at all in that bug.

- Grant