nvc0: fix null ptr deref on fermi due to debug changes.
What does this MR do and why?
nvc0: fix null ptr deref on fermi due to debug changes.
Not everyone has a copy class, so don't dereference it if it's not
set.
Pointed out on irc by Armada
Fixes: 65092ab1a552 ("nouveau/nvc0: add support for using common pushbuf dumper")