drm/nouveau: Revert "bus: remove cpu_coherent flag"

This reverts commit aff51175.

The commit caused fence timeouts within nvc0_screen_destroy and most likely
other places as well.

The most obvious effect is, that userspace processes take minutes to
actually quit.

Signed-off-by: Karol Herbst <karolherbst@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>