nouveau: fixup scanout enable in nvc0_pm

commit 44b9f44e upstream.

Fixes screen being black after changing performance level.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>