drm: fix drm_format_helper.h kernel-doc warnings

As the documentation says, all the fields are considered private. Mark
them private also for kernel-doc to silence warnings.
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/6dc5b1df34abc10d416c2db5b390440cf56e21ce.1709749576.git.jani.nikula@intel.com

include/drm/drm_format_helper.h

@@ -25,6 +25,7 @@ struct iosys_map;
  * All fields are considered private.
  */
 struct drm_format_conv_state {
+	/* private: */
 	struct {
 		void *mem;
 		size_t size;