drm/stm: ltdc: Use simple encoder

STM ltdc driver uses an empty implementation for its encoder.
Replace the code with the generic simple encoder.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-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/20210302175700.28640-1-jagan@amarulasolutions.com