drm/nouveau/clk/gk20a: only restore divider to 1:1 if needed

Only restore the 1:1 divider if it is not set already. Also use the
proper masks for this operation and add a second write as done in the
Android code.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>