drm/i915: add missing SDVO bits for interlaced modes on ILK

This was pointed by Jesse Barnes. The code now seems to follow the
specification but I don't have an SDVO device to really test this.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>