nouveau: flip DEBUG -> !NDEBUG

nouveau: flip DEBUG -> !NDEBUG

The meson conversion chose to change the meaning of DEBUG to "used for
debugging" to be "used for expensive things for debugging", primarily
for nir_validate. Flip things over so that we get nice things with
optimizations enabled.

While we're at it, also kill off nouveau_statebuf.h which is unused (and
has a mention of DEBUG which is how I found it).

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Karol Herbst <kherbst@redhat.com>