drm/nouveau: handle fifo pusher errors better

The most important part of this change is that we now instruct PFIFO to
drop all pending fetches, rather than attempting to skip a single dword
and hope that things would magically sort themselves out - they usually
don't, and we end up with PFIFO being completely hung.

This commit also adds somewhat more useful logging when these exceptions
occur.

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