drm/nouveau/mpeg: mthd returns true on success now

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Fixes: 590801c1 ("drm/nouveau/mpeg: remove dependence on namedb/engctx lookup")
Cc: stable@vger.kernel.org # v4.3+
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>