drm/sti: do not set gdp pixel clock rate if mode is not set

Fix a division by 0 case : in some cases, when the GDP plane is being
disabled atomic_check() is called with "mode->clock = 0".
In that case, do not set parent and pixel clock rate.
Signed-off-by: Vincent Abriou <vincent.abriou@st.com>

drivers/gpu/drm/sti/sti_gdp.c

@@ -648,7 +648,7 @@ static int sti_gdp_atomic_check(struct drm_plane *drm_plane,
 	}
 
 	/* Set gdp clock */
-	if (gdp->clk_pix) {
+	if (mode->clock && gdp->clk_pix) {
 		struct clk *clkp;
 		int rate = mode->clock * 1000;
 		int res;