drm/nouveau/debugfs: Fix check of pm_runtime_get_sync failure

pm_runtime_get_sync returns negative on failure.

Fixes: eaeb9010 ("drm/nouveau/debugfs: Wake up GPU before doing any reclocking")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>