[Nouveau] Interrupt setting

[Luca Barbieri]

> Since you create one fence object for each pushbuf, I thought that we can
> synchronize only with last the command.
> Not sure if my assumption is correct...
All the commands in the pushbuffer are executed sequentially and the
fence setting command is written at the end of the pushbuffer, so when
the fence register is updated, all commands will have been already
executed.

However, this does indeed mean that to wait for the completion of any
command, we also need to wait for the completion of all the other
commands in the pushbuffer sent in the pushbuf ioctl call.

This could be improved, but I doubt it is worth the significant
additional complexity and CPU performance costs. Userspace can always
send commands in smaller chunks if it wants to.