drm/ttm: minor cleanup to move to system

resource free already sets the domain to system, and old_mem
isn't really needed.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201020010319.1692445-3-airlied@gmail.com