drm/nouveau: Use the software object for fencing.

This should avoid a race condition on nv0x, if we're doing it with
actual PGRAPH objects and a there's a fence within the FIFO DMA fetch
area when a context switch kicks in.

In that case we get an ILLEGAL_MTHD interrupt as expected, but the
values in PGRAPH_TRAPPED_ADDR aren't calculated correctly and they're
almost useless (e.g. you can see ILLEGAL_MTHDs for the now inactive
channel, with a wrong offset/data pair).

Signed-off-by: Francisco Jerez <currojerez@riseup.net>