drm/i915: use VBT to determine whether to enumerate the VGA port

Some platforms may not have it, and enumerating it is both confusing and
time consuming due to the hotplug and DDC probing.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>