drm/i915: Enable the overlay right after primary and cursor planes

Again follow the same sequence for all generations, because doing
otherwise just doesn't make sense.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>