stm: ltdc: fix two incorrect NULL checks on list iterator

The two bugs are here:
	if (encoder) {
	if (bridge && bridge->timings)

The list iterator value 'encoder/bridge' will *always* be set and
non-NULL by drm_for_each_encoder()/list_for_each_entry(), so it is
incorrect to assume that the iterator value will be NULL if the
list is empty or no element is found.

To fix the bug, use a new variable '*_iter' as the list iterator,
while use the old variable 'encoder/bridge' as a dedicated pointer
to point to the found element.

Cc: stable@vger.kernel.org
Fixes: 99e36044 ("drm/stm: Fix bus_flags handling")
Signed-off-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>
Acked-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>
Signed-off-by: Philippe Cornu <philippe.cornu@foss.st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220327055355.3808-1-xiam0nd.tong@gmail.com

drivers/gpu/drm/stm/ltdc.c

@@ -869,8 +869,8 @@ static void ltdc_crtc_mode_set_nofb(struct drm_crtc *crtc)
 	struct drm_device *ddev = crtc->dev;
 	struct drm_connector_list_iter iter;
 	struct drm_connector *connector = NULL;
-	struct drm_encoder *encoder = NULL;
-	struct drm_bridge *bridge = NULL;
+	struct drm_encoder *encoder = NULL, *en_iter;
+	struct drm_bridge *bridge = NULL, *br_iter;
 	struct drm_display_mode *mode = &crtc->state->adjusted_mode;
 	u32 hsync, vsync, accum_hbp, accum_vbp, accum_act_w, accum_act_h;
 	u32 total_width, total_height;
@@ -880,15 +880,19 @@ static void ltdc_crtc_mode_set_nofb(struct drm_crtc *crtc)
 	int ret;
 
 	/* get encoder from crtc */
-	drm_for_each_encoder(encoder, ddev)
-		if (encoder->crtc == crtc)
+	drm_for_each_encoder(en_iter, ddev)
+		if (en_iter->crtc == crtc) {
+			encoder = en_iter;
 			break;
+		}
 
 	if (encoder) {
 		/* get bridge from encoder */
-		list_for_each_entry(bridge, &encoder->bridge_chain, chain_node)
-			if (bridge->encoder == encoder)
+		list_for_each_entry(br_iter, &encoder->bridge_chain, chain_node)
+			if (br_iter->encoder == encoder) {
+				bridge = br_iter;
 				break;
+			}
 
 		/* Get the connector from encoder */
 		drm_connector_list_iter_begin(ddev, &iter);