drm/stm: checkpatch strict minor updates

Minor fixes detected with "scripts/checkpatch.pl --strict"

Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1509013473-18365-1-git-send-email-philippe.cornu@st.com