drm/nouveau/fifo: add chan start()/stop()

- nvkm_chan_error() built on top, stops channel and sends 'killed' event
- removes an odd double-bashing of channel enable regs on kepler and up
- pokes doorbell on turing and up, after enabling channel

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>