drm: sti: fix delay in VTG programming

The HDMI path introduce a delay of 6 pixels.
This delay should be take into account while programming
VTG for the HDMI. Without this delay, the HDMI active
window area is shift of 6 pixel on the right.

Set also timing for DVO output.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>