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: Liviu Dudau <Liviu.Dudau@arm.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1464729075-22243-2-git-send-email-daniel.vetter@ffwll.ch