drm/sti: vtg fix CEA-861E video format timing error

HDMI analyzer tests showed that Vsync and Hsync signal were not
compliant with the HDMI protocol.

HDMI_DELAY should be taken into account in the VTG Vsync
programming to reflect the 6 pixels shift introduced in the VTG
Hsync programming.

Signed-off-by: Vincent Abriou <vincent.abriou@st.com>