drm/doc: Capture TODO for deferred fbdev setup

Also became a bit a FAQ recently.

Cc: John Stultz <john.stultz@linaro.org
Cc: Thierry Reding <thierry.reding@gmail.com>
Acked-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170226193851.3245-2-daniel.vetter@ffwll.ch