drm/nouveau/clk/gf100-: Clean up PLL locking test

Corresponds with GT215. Don't rely on the lock test logic being
unconditionally enabled, and disable test logic when done (presumably
to save power).

v2: Remove warning, nvkm_msec already warns on time-out

Signed-off-by: Roy Spliet <nouveau@spliet.org>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>