[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.

- Grant