drm/nouveau/runpm: fix module unload

Use the new vga_switcheroo_fini_domain_pm_ops function
to unregister the pm ops.

Based on a patch from:
Pali Rohár <pali.rohar@gmail.com>

bug:
https://bugzilla.kernel.org/show_bug.cgi?id=84431



Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Cc: Ben Skeggs <bskeggs@redhat.com>