drm: make of_drm_find_panel also depend on CONFIG_DRM_PANEL

For drm_of_find_panel_or_bridge() added in the next commit, an empty
version of of_drm_find_panel is needed for !CONFIG_DRM_PANEL.
Signed-off-by: NRob Herring <robh@kernel.org>
Reviewed-by: NEric Anholt <eric@anholt.net>
Signed-off-by: NSean Paul <seanpaul@chromium.org>

include/drm/drm_panel.h

@@ -192,7 +192,7 @@ void drm_panel_remove(struct drm_panel *panel);
 int drm_panel_attach(struct drm_panel *panel, struct drm_connector *connector);
 int drm_panel_detach(struct drm_panel *panel);
 
-#ifdef CONFIG_OF
+#if defined(CONFIG_OF) && defined(CONFIG_DRM_PANEL)
 struct drm_panel *of_drm_find_panel(const struct device_node *np);
 #else
 static inline struct drm_panel *of_drm_find_panel(const struct device_node *np)