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

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

The first active pixel of a line is defined by HDMI_ACTIVE_VID_XMIN.
The last active pixel of a line is defined by HDMI_ACTIVE_VID_XMAX.

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