drm/ttm: start allowing drivers to use new takedown path (v2)

Allow the takedown path callback to be optional as well.

v2: use fini for range manager

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200804025632.3868079-27-airlied@gmail.com