drm/i915: set the DDI sync polarity bits

During my tests, everything worked even if the wrong polarity was set.
Still, we should try to set the correct values.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>