drm/atomic-helper: Fixup kerneldoc for drm_atomic_helper_legacy_gamma_set

kernel-doc is unhappy, appease it.

Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Acked-by: NLiviu Dudau <Liviu.Dudau@arm.com>
Acked-by: NJani Nikula <jani.nikula@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1464729075-22243-2-git-send-email-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_atomic_helper.c

@@ -2920,7 +2920,7 @@ EXPORT_SYMBOL(drm_atomic_helper_connector_destroy_state);
  * @red: red correction table
  * @green: green correction table
  * @blue: green correction table
- * @start:
+ * @start: first entry, must always be 0
  * @size: size of the tables
  *
  * Implements support for legacy gamma correction table for drivers