drm/nouveau/kms: Implement KDB debug hooks for nouveau KMS.

Tested on nv50 and nv04 HW.

Signed-off-by: Chris Ball <cjb@laptop.org>
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
CC: Jesse Barnes <jbarnes@virtuousgeek.org>
CC: dri-devel@lists.freedesktop.org
Signed-off-by: Dave Airlie <airlied@redhat.com>