drm: sti: fix W=1 warnings

Fix warnings when W=1.
No code changes, only clean up in sti internal structures and functions
descriptions.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Reviewed-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190909101254.24191-1-benjamin.gaignard@st.com