drm/ttm: Fix memory type manager debug information printing

System memory type doesn't have a drm_mm manager associated to
it. This patch avoid trying to call drm_mm_debug on unitialized
drm_mm when printing debug info on the system memory manager.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>