drm/ttm: move last binding into the drivers.

This moves the call to tt binding into the driver move,
and drops the driver callback.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201020010319.1692445-8-airlied@gmail.com