drm/bridge/synopsys: dsi: missing post disable

Sometime the post_disable function is missing (not registered).
Signed-off-by: NYannick Fertré <yannick.fertre@st.com>
Reviewed-by: NPhilippe Cornu <philippe.cornu@st.com>
Signed-off-by: NNeil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1579602296-7683-1-git-send-email-yannick.fertre@st.com

drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi.c

@@ -824,7 +824,8 @@ static void dw_mipi_dsi_bridge_post_disable(struct drm_bridge *bridge)
 	 * This needs to be fixed in the drm_bridge framework and the API
 	 * needs to be updated to manage our own call chains...
 	 */
-	dsi->panel_bridge->funcs->post_disable(dsi->panel_bridge);
+	if (dsi->panel_bridge->funcs->post_disable)
+		dsi->panel_bridge->funcs->post_disable(dsi->panel_bridge);
 
 	if (phy_ops->power_off)
 		phy_ops->power_off(dsi->plat_data->priv_data);