drm/stm: ltdc: disable all layers before crtc shutdown

All plans must be disabled before the CRTC shutdown helping
the crtc to restart from a clean situation (without unwanted
planes already enable).

Signed-off-by: Yannick Fertre <yannick.fertre@foss.st.com>
Signed-off-by: Philippe Cornu <philippe.cornu@foss.st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220603134151.591997-1-yannick.fertre@foss.st.com