drm/sti: force cursor CLUT fetch

It may happen that the cursor is displayed with wrong colors which can
be explained by a CLUT wrongly fetched at the first display.
Fetching the CLUT at each commit (=move) ensures that the right colors
are used, at least from the first cursor move.

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Reviewed-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Reviewed-by: Vincent Abriou <vincent.abriou@st.com>