drm/tegra: dc: Use atomic clock state in modeset

drm/tegra: dc: Use atomic clock state in modeset

All clock state is now stored in the display controller's atomic state,
so the output drivers no longer need to call back into the display
controller driver to set up the clock. This is also required to make
sure no hardware changes are made before validating a configuration.

Signed-off-by: Thierry Reding <treding@nvidia.com>