drm/i915/vlv: use W_SYNC_SHIFT for interlaced modes on VLV

This makes HDMI testers happier on VLV platforms.  It may be that we
need it for any non-SVO platform, but I don't have any tests to back
that up, so I'm leaving other pre-ILK platforms alone for now.

Tested-by: "Clint Taylor <clinton.a.taylor@intel.com&gt;">
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=74964
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>