drm/nouveau: map pages using DMA API

The DMA API is the recommended way to map pages no matter what the
underlying bus is. Use the DMA functions for page mapping and remove
currently existing wrappers.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>