drm/nouveau/clk/gk20a: split gk20a_clk_new()

This allows to instanciate drivers that use the same logic as gk20a with
different parameters.

Add a constructor function to allow other chips that inherit from this
clock to easily initialize its members

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