drm/atomic: Make disable_all helper fully disable the crtc.

It seems that nouveau requires this, so best to do this in the helper.
This allows nouveau to use the atomic suspend helper.

Cc: nouveau@lists.freedesktop.org
Acked-by: Ben Skeggs <bskeggs@redhat.com> #irc
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1487685102-31991-2-git-send-email-maarten.lankhorst@linux.intel.com