drm/nouveau/clk/gk20a: parameterize PLL settings

Make functions manipulating PLL settings take them as an argument,
instead of assuming we want to work on the copy in the gk20a_clk
structure. This makes these functions more flexible, which we will need
in GM20B.

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