drm/nouveau: workaround semaphore hw bug causing unnecessary interrupts

The HW will only accept the DMA_FROM_MEMORY class for DMA_SEMAPHORE without
asking the driver to intervene.

It appears that semaphores will work correctly even without DMA_IN_MEMORY,
so lets avoid the large amount of interrupts generated by x-chan sync.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>