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 <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Link: http://patchwork.freedesktop.org/patch/msgid/[e-mail]

include/drm/drm_panel.h

···
 #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;