drm/ttm: flip tt destroy ordering.

Call the driver first and have it call the common code cleanup.

This is useful later to fix unbind.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200917043040.146575-3-airlied@gmail.com