drm/drm_panel: Fix EXPORT of drm_panel_of_backlight() one more time

The initial commit followed by the fix didn't
take into consideration the case:

CONFIG_DRM_PANEL=y
CONFIG_BACKLIGHT_CLASS_DEVICE=m
CONFIG_DRM_I915=y

where symbol devm_of_find_backlight() is not reachable from DRM subsystem.
Quick fix is to avoid drm_panel_of_backlight() from exporting in such case.

Fixes: 907aa265fd ("drm/drm_panel: fix EXPORT of drm_panel_of_backlight")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191217140721.42432-1-andriy.shevchenko@linux.intel.com

drivers/gpu/drm/drm_panel.c

@@ -302,7 +302,7 @@ struct drm_panel *of_drm_find_panel(const struct device_node *np)
 EXPORT_SYMBOL(of_drm_find_panel);
 #endif
 
-#if IS_ENABLED(CONFIG_BACKLIGHT_CLASS_DEVICE)
+#if IS_REACHABLE(CONFIG_BACKLIGHT_CLASS_DEVICE)
 /**
  * drm_panel_of_backlight - use backlight device node for backlight
  * @panel: DRM panel

include/drm/drm_panel.h

@@ -198,7 +198,7 @@ static inline struct drm_panel *of_drm_find_panel(const struct device_node *np)
 }
 #endif
 
-#if IS_ENABLED(CONFIG_BACKLIGHT_CLASS_DEVICE)
+#if IS_REACHABLE(CONFIG_BACKLIGHT_CLASS_DEVICE)
 int drm_panel_of_backlight(struct drm_panel *panel);
 #else
 static inline int drm_panel_of_backlight(struct drm_panel *panel)