drm/sti: adapt YUV colorspace in display pipeline

Use BT601 for SD/ED resolution and BT709 for HD resolution

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>