drm/stm: ltdc: Drop format_mod_supported function

The "drm_plane_funcs.format_mod_supported" can be removed in favor of
the default implementation.

Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Reviewed-by: Yannick Fertre <yannick.fertre@foss.st.com>
Tested-by: Yannick Fertre <yannick.fertre@foss.st.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Tested-by: Jagan Teki <jagan@amarulasolutions.com> # i.Core STM32MP1
Signed-off-by: Philippe Cornu <philippe.cornu@foss.st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211222090552.25972-7-jose.exposito89@gmail.com