drm/sti: run gdp init sequence only once

Do not rely on plane->status to define whether this is the first update
but rather check for gdp->vtg.
This avoids multiple and unwanted calls to sti_vtg_register_client()
which breaks the kernel scheduler.

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Acked-by: Vincent Abriou <vincent.abriou@st.com>