drm/nouveau: Remove nouveau_drm_debugfs_cleanup()

drm_debugfs_cleanup() now removes all minor->debugfs_list entries
automatically, so the drm_driver.debugfs_cleanup callback is not
needed. Additionally it uses debugfs_remove_recursive() to clean
up the debugfs files, so no need for adding fake drm_info_node
entries.

Cc: bskeggs@redhat.com
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170126225621.12314-11-noralf@tronnes.org