drm/sti: Do not export symbols

None of these exported symbols are used outside of the drm-sti driver,
so there is no reason to export them.

Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: Vincent Abriou <vincent.abriou@st.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Vincent Abriou <vincent.abriou@st.com>