drm/nouveau/falcon: avoid touching registers if engine is off

[ Upstream commit a5176a4cb85bb6213daadf691097cf411da35df2 ]

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108980


Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>