drm/of: make drm_of.h self-contained

Include <linux/err.h> for ERR_PTR.
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/e9b3ea30984b913f6e32ed35e0575438dff8278c.1709749576.git.jani.nikula@intel.com

include/drm/drm_of.h

@@ -2,6 +2,7 @@
 #ifndef __DRM_OF_H__
 #define __DRM_OF_H__
 
+#include <linux/err.h>
 #include <linux/of_graph.h>
 #if IS_ENABLED(CONFIG_OF) && IS_ENABLED(CONFIG_DRM_PANEL_BRIDGE)
 #include <drm/drm_bridge.h>