drm/i915/skl: Correctly updating sprite wm parameter

The pipe wm parameters is not correctly updated with sprite parameters
because it copies them for each plane from plane_list to the sprite
offset in pipe wm parameters. Since plane_list also contains primary and
cursor planes, we end up updating wrong params for sprites.

Signed-off-by: Sonika Jindal <sonika.jindal@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>