drm/panel: make drm_panel.h self-contained

Fix build warning if drm_panel.h is built with CONFIG_OF=n or
CONFIG_DRM_PANEL=n and included without the prerequisite err.h:

./include/drm/drm_panel.h: In function ‘of_drm_find_panel’:
./include/drm/drm_panel.h:203:9: error: implicit declaration of function ‘ERR_PTR’ [-Werror=implicit-function-declaration]
  return ERR_PTR(-ENODEV);
         ^~~~~~~
./include/drm/drm_panel.h:203:9: error: returning ‘int’ from a function with return type ‘struct drm_panel *’ makes pointer from integer without a cast [-Werror=int-conversion]
  return ERR_PTR(-ENODEV);
         ^~~~~~~~~~~~~~~~

Fixes: 5fa8e4a2 ("drm/panel: Make of_drm_find_panel() return an ERR_PTR() instead of NULL")
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Sean Paul <sean@poorly.run>
Cc: Boris Brezillon <bbrezillon@kernel.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190718161507.2047-2-sam@ravnborg.org

include/drm/drm_panel.h

@@ -24,6 +24,7 @@
 #ifndef __DRM_PANEL_H__
 #define __DRM_PANEL_H__
 
+#include <linux/err.h>
 #include <linux/errno.h>
 #include <linux/list.h>