drm/nouveau: Check dsm on switcheroo unregister

Currently vga_switcheroo_unregister_handler is called unconditionally when
nouveau is unloaded, even when nouveau never registered a handler. This
interferes with other switcheroo handlers, as vga_switcheroo doesn't check who
called unregister_handler, but simply unregisters the current handler. This
patch adds a check so unregister is only called if a handler was registered by
nouveau before.

Signed-off-by: Andreas Heider <andreas@meetr.de>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>