drm/radeon: Don't register Thunderbolt eGPU with vga_switcheroo

An external Thunderbolt GPU can neither drive the laptop's panel nor be
powered off by the platform, so there's no point in registering it with
vga_switcheroo.  In fact, when the external GPU is runtime suspended,
vga_switcheroo will cut power to the internal discrete GPU, resulting in
a lockup.  Moreover AMD's Windows driver special-cases Thunderbolt as
well.

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Link: http://patchwork.freedesktop.org/patch/msgid/72d8a9645aece3eff44e116303f0fec8be061c88.1489145162.git.lukas@wunner.de