drm/doc: Fix missing @ctx documentation

Forgot to add this :(

Fixes: 1931529448bc ("drm: Add acquire ctx parameter to ->plane_disable")
Cc: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170413074007.7620-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_plane_helper.c

@@ -381,6 +381,7 @@ EXPORT_SYMBOL(drm_primary_helper_update);
 /**
  * drm_primary_helper_disable() - Helper for primary plane disable
  * @plane: plane to disable
+ * @ctx: lock acquire context, not used here
  *
  * Provides a default plane disable handler for primary planes.  This is handler
  * is called in response to a userspace SetPlane operation on the plane with a