drm: make drm_panel.h self-contained

-ENOSYS and -EINVAL are referenced in some static inline functions.
of_drm_find_pane() takes a pointer to struct device_node.

Make this header self-contained to not depend on specific include
order.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1493009447-31524-2-git-send-email-yamada.masahiro@socionext.com

include/drm/drm_panel.h

@@ -24,8 +24,10 @@
 #ifndef __DRM_PANEL_H__
 #define __DRM_PANEL_H__
 
+#include <linux/errno.h>
 #include <linux/list.h>
 
+struct device_node;
 struct drm_connector;
 struct drm_device;
 struct drm_panel;