drm/msm: dpu: Don't set legacy plane->crtc pointer

It causes a WARN in drm_atomic_get_plane_state(), and is not used by
atomic (or dpu).

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Rob Clark <robdclark@gmail.com>