drm/stm: ltdc: reset plane transparency after plane disable

The plane's opacity should be reseted while the plane
is disabled. It prevents from seeing a possible global
or layer background color set earlier.

Signed-off-by: Yannick Fertre <yannick.fertre@foss.st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240712131344.98113-1-yannick.fertre@foss.st.com


Signed-off-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>