drm/doc: kerneldoc for quirk_addfb_prefer_xbgr_30bpp

Shuts up warning noise.

Reviewed-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181004202446.22905-22-daniel.vetter@ffwll.ch
diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
index 928e417..d643d26 100644
--- a/include/drm/drm_mode_config.h
+++ b/include/drm/drm_mode_config.h
@@ -809,6 +809,13 @@
 
 	/* dumb ioctl parameters */
 	uint32_t preferred_depth, prefer_shadow;
+
+	/**
+	 * @quirk_addfb_prefer_xbgr_30bpp:
+	 *
+	 * Special hack for legacy ADDFB to keep nouveau userspace happy. Should
+	 * only ever be set by the nouveau kernel driver.
+	 */
 	bool quirk_addfb_prefer_xbgr_30bpp;
 
 	/**