drm/sti: fix dvo data_enable signal

Modify AWG algorithm in order to handle more than 1023 lines

Signed-off-by: Bich Hemon <bich.hemon@st.com>
Reviewed-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Reviewed-by: Vincent Abriou <vincent.abriou@st.com>