drm/ttm: tt destroy move null check to outer function.

This just makes things easier later.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200915024007.67163-4-airlied@gmail.com