drm/sun4i: tcon: fix check of tcon->panel null pointer

Since tcon->panel is a pointer returned by of_drm_find_panel() need to
check if it is not NULL, hence a valid pointer.
IS_ERR() instead checks return error values, not NULL pointers.

Substitute "if (!IS_ERR(tcon->panel))" with "if (tcon->panel)".
Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181005215951.99003-1-giulio.benetti@micronovasrl.com

drivers/gpu/drm/sun4i/sun4i_lvds.c

@@ -75,7 +75,7 @@ static void sun4i_lvds_encoder_enable(struct drm_encoder *encoder)
 
 	DRM_DEBUG_DRIVER("Enabling LVDS output\n");
 
-	if (!IS_ERR(tcon->panel)) {
+	if (tcon->panel) {
 		drm_panel_prepare(tcon->panel);
 		drm_panel_enable(tcon->panel);
 	}
@@ -88,7 +88,7 @@ static void sun4i_lvds_encoder_disable(struct drm_encoder *encoder)
 
 	DRM_DEBUG_DRIVER("Disabling LVDS output\n");
 
-	if (!IS_ERR(tcon->panel)) {
+	if (tcon->panel) {
 		drm_panel_disable(tcon->panel);
 		drm_panel_unprepare(tcon->panel);
 	}

drivers/gpu/drm/sun4i/sun4i_rgb.c

@@ -135,7 +135,7 @@ static void sun4i_rgb_encoder_enable(struct drm_encoder *encoder)
 
 	DRM_DEBUG_DRIVER("Enabling RGB output\n");
 
-	if (!IS_ERR(tcon->panel)) {
+	if (tcon->panel) {
 		drm_panel_prepare(tcon->panel);
 		drm_panel_enable(tcon->panel);
 	}
@@ -148,7 +148,7 @@ static void sun4i_rgb_encoder_disable(struct drm_encoder *encoder)
 
 	DRM_DEBUG_DRIVER("Disabling RGB output\n");
 
-	if (!IS_ERR(tcon->panel)) {
+	if (tcon->panel) {
 		drm_panel_disable(tcon->panel);
 		drm_panel_unprepare(tcon->panel);
 	}

drivers/gpu/drm/sun4i/sun4i_tcon.c

@@ -555,7 +555,7 @@ static void sun4i_tcon0_mode_set_rgb(struct sun4i_tcon *tcon,
 	 * Following code is a way to avoid quirks all around TCON
 	 * and DOTCLOCK drivers.
 	 */
-	if (!IS_ERR(tcon->panel)) {
+	if (tcon->panel) {
 		struct drm_panel *panel = tcon->panel;
 		struct drm_connector *connector = panel->connector;
 		struct drm_display_info display_info = connector->display_info;