drm/gk20a/clk: fix max VCO value

For some reason max_vco was set to a lower value that it can support,
which prevented some clock states to be applied. Fix this by setting it
to the same value as downstream.

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