drm/i915: dvo_ch7xxx: fix vsync polarity setting

This fixes a typo which set the wrong vsync and possibly also hsync
polarity for any modes with positive vsync polarity.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>