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: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170620094228.4757-1-daniel.vetter@ffwll.ch