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: 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