drm/nouveau/nvif: fix potential double-free

DRM cleanup paths unconditionally call nvif_mmu_dtor() for clients,
which would result in a double-free if nvif_mmu_ctor()'d previously
failed.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230525003106.3853741-9-skeggsb@gmail.com