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>

drivers/gpu/drm/sti/sti_hqvdp.c

@@ -1037,7 +1037,7 @@ static int sti_hqvdp_atomic_check(struct drm_plane *drm_plane,
 	src_w = state->src_w >> 16;
 	src_h = state->src_h >> 16;
 
-	if (!sti_hqvdp_check_hw_scaling(hqvdp, mode,
+	if (mode->clock && !sti_hqvdp_check_hw_scaling(hqvdp, mode,
 						       src_w, src_h,
 						       dst_w, dst_h)) {
 		DRM_ERROR("Scaling beyond HW capabilities\n");