ry corruption when pgt allocation fails

If we return freed vm, nouveau_drm_open will happily call nouveau_cli_destroy,
which will try to free it again.

Reported-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
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