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