drm/nouveau/clk/gk20a: improve MNP programming

Split the MNP programming function into two functions for the cases
where we allow sliding or not, instead of making it take a parameter for
this. This results in less conditionals in the code and makes it easier
to read.

Also make the MNP programming functions take the PLL parameters as
arguments, and move bits of code to more relevant places (previous
programming tended to be just-in-time, which added more conditionnals in
the code).

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