drm/i915: Use intel_wm_plane_visible() on VLV/CHV as well

VLV/CHV don't have double buffered watermarks so they need to consider
the cursor visibility as a special case just like ILK-BDW. Let's use
the helper we have for that.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170303151928.23053-4-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>