drm/nouveau/clk: support for non-BIOS pstates

Make nouveau_clock_create() take new two optional arguments: an array
of pstates and its size. When these are specified,
nouveau_clock_create() will use the provided pstates instead of
probing them using the BIOS.

This is useful for platforms which do not provide a BIOS, like Tegra.

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