drm/sti: use valid video mode

In atomic mode the crtc_xxx (eg crtc_hdisplay) members of the mode
structure may be unset before calling atomic_check/commit for planes.
Instead of, use xxx members which are actually set.

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