clk: tegra: Optimize PLLX restore on Tegra20/30

There is no need to re-configure PLLX if its configuration in unchanged
on return from suspend / cpuidle, this saves 300us if PLLX is already
enabled (common case for cpuidle).

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>