drm: sti: remove useless fields from vtg structure

Drivers are registered on platform bus so the private list
could be replace by a call to of_find_device_by_node().
Changing this also makes dev, np and link fields useless
in vtg structure.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1510146255-24982-1-git-send-email-benjamin.gaignard@linaro.org