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 <sam@ravnborg.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Fixes: 152dbde ("drm/panel: add backlight support")
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: Maxime Ripard <maxime.ripard@bootlin.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
Link: https://patchwork.freedesktop.org/patch/msgid/20191210194758.24087-1-sam@ravnborg.org

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