drm/nouveau/volt/gk20a: share reusable members & functions

The CVB calculation and voltage setting functions can be reused for the
future chips. So move the declaration to gk20a.h.

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