drm/crtc-helper: disable_unused_functions really isn't for atomic

Rockchip just blew up here on testing, because I removed some "is this
crtc already disabled/enabled" state tracking from callbacks (not needed
with atomic). Turns out that was needed to work around rockchip still
calling legacy helper code.

Since me explaining on irc/mailing-list plus kerneldoc isn't enough,
be more verbose and add dmesg output. Not that anyone actually reads that,
either.

Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1465388359-8070-26-git-send-email-daniel.vetter@ffwll.ch