drm/drm_panel: fix EXPORT of drm_panel_of_backlight

Fix link failure for module builds of panels.
The conditional compilation around drm_panel_of_backlight()
was wrong for a module build.
Fix it using IS_ENABLED().

Signed-off-by: Sam Ravnborg <[email protected]>
Reviewed-by: Linus Walleij <[email protected]>
Fixes: 152dbdeab1 ("drm/panel: add backlight support")
Cc: Sam Ravnborg <[email protected]>
Cc: Laurent Pinchart <[email protected]>
Cc: Thierry Reding <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
Cc: Maxime Ripard <[email protected]>
Cc: Sean Paul <[email protected]>
Cc: David Airlie <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Maxime Ripard <[email protected]>
Cc: [email protected]
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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
 
-#ifdef CONFIG_BACKLIGHT_CLASS_DEVICE
+#if IS_ENABLED(CONFIG_BACKLIGHT_CLASS_DEVICE)
 /**
  * drm_panel_of_backlight - use backlight device node for backlight
  * @panel: DRM panel