drm/i915: Eliminate usage of plane_wm_parameters from ILK-style WM code (v2)

Just pull the info out of the plane state structure rather than staging
it in an additional structure.

v2: Add 'visible' condition to sprites_scaled so that we don't limit the
    WM level when the sprite isn't enabled.  (Ville)

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by(v1): Ander Conselvan de Oliveira <conselvan2@gmail.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>