nouveau: add coherent BO attribute

Add a flag allowing Nouveau to specify that an object should be coherent
at allocation time. This is required for some class of objects like
fences which are randomly-accessed by both the CPU and GPU. This flag
instructs the kernel driver to make sure the object remains coherent
even on architectures for which coherency is not guaranteed by the bus.

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