drm/nouveau/ttm: remove special handling of coherent objects

TTM-allocated coherent objects were populated using the DMA API and
accessed using the mapping it returned to workaround coherency
issues. These issues seem to have been solved, thus remove this extra
case to handle and use the regular kernel mapping functions.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>