apple-gmux: Don't switch external DP port on 2011+ MacBook Pros

On MacBook Pros introduced 2011 and onward, external DP ports are
combined DP/Thunderbolt ports that are no longer fully switchable
between GPUs, they can only be driven by the discrete GPU.

More specifically, the Main Link pins (which transport the actual video
and audio streams) are soldered to the discrete GPU, whereas the AUX
Channel pins are switchable. Because the integrated GPU is missing the
Main Link, external displays appear to it as phantoms which fail to
link-train.

Force the AUX channel to the discrete GPU on these models to avoid any
confusion. Document the switching policy implemented by this commit.

Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Link: http://patchwork.freedesktop.org/patch/msgid/4d1fcc92d1960049e2cff997fbd2d74e45e84e49.1489145162.git.lukas@wunner.de