drm/ttm: further cleanup ttm_mem_reg handling

Stop touching the backend private pointer alltogether and
make sure we never put the same mem twice by.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Madhav Chauhan <madhav.chauhan@amd.com>
Link: https://patchwork.freedesktop.org/patch/375613/