drm/i915: Move vlv wms from crtc->wm_state to crtc->wm.active.vlv

In an effort to make the vlv/chv wm code look and behave more like the
ilk+ code, let's move the current active wms next to the
corresponding ilk wms.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170302171508.1666-4-ville.syrjala@linux.intel.com