drm/nouveau/clk/gk20a: rename enable/disable functions

gk20a_pllg_disable() is only used in the context of gk20a_clk_fini().
Move its body there and rename _gk20a_pllg_enable() and
_gk20a_pllg_disable() to non-underscored versions.

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