drm/nouveau: synchronize BOs when required

On architectures for which access to GPU memory is non-coherent,
caches need to be flushed and invalidated explicitly when BO control
changes between CPU and GPU.

This patch adds buffer synchronization functions which invokes the
correct API (PCI or DMA) to ensure synchronization is effective.

Based on the TTM DMA cache helper patches by Lucas Stach.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>