drm/sti: do not check hw scaling if mode is not set

Fix a division by 0 case : in some cases, when the HQVDP plane is being
disabled atomic_check() is called with "mode->clock = 0".
In that case, do not check for scaling capabilities.

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