drm/ttm: remove cpu_address member from ttm_tt

Patch 3d50d4dc exposed the CPU address of DMA-allocated pages as
returned by dma_alloc_coherent because Nouveau on Tegra needed it.

This is not required anymore - as there were no other users for it,
remove it and save some memory for everyone.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>