drm/stm: ltdc: fix various coding-style warnings

Fix issues reported by checkpatch.pl:
- Braces {} should be used on all arms
- Blank lines

Signed-off-by: Yannick Fertre <yannick.fertre@foss.st.com>
Acked-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>
Signed-off-by: Philippe Cornu <philippe.cornu@foss.st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220603134334.592805-1-yannick.fertre@foss.st.com