drm/i915: disable cloning on sdvo

After the recent pile of disable-cloning patches, e.g.

commit e3b86d69
Author: Egbert Eich <eich@suse.de>
Date:   Sat Oct 13 14:30:15 2012 +0200

    DRM/i915: Don't clone SDVO LVDS with analog

and a bug report from Chris Wilson indicating that cloning doesn't
even work for DVI-SDVO and native VGA, let's just disable cloning on
sdvo encoders completely.

v2: Update the comment in the code as discussed with Paulo Zanoni.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=29259


Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>