drm: More links for gamma support helpers

It's not obvious that when using the new color manager stuff you still
need to wire up the legacy helper to get legacy LUT support. Improve
this with more links. drm_crtc_funcs->gamma_set already explains this
properly.

Cc: Peter Rosin <peda@axentia.se>
Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: NBoris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170620094228.4757-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_color_mgmt.c

@@ -128,6 +128,9 @@ EXPORT_SYMBOL(drm_color_lut_extract);
  * optional. The gamma and degamma properties are only attached if
  * their size is not 0 and ctm_property is only attached if has_ctm is
  * true.
+ *
+ * Drivers should use drm_atomic_helper_legacy_gamma_set() to implement the
+ * legacy &drm_crtc_funcs.gamma_set callback.
  */
 void drm_crtc_enable_color_mgmt(struct drm_crtc *crtc,
 				uint degamma_lut_size,