drm/ttm: merge ttm_dma_tt back into ttm_tt

It makes no difference to kmalloc if the structure
is 48 or 64 bytes in size.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/396950/