drm/ttm: move unbind into the tt destroy.

This moves unbind into the driver side on destroy paths.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200917043040.146575-4-airlied@gmail.com