drm/tegra: dc: Use base atomic state helpers

drm/tegra: dc: Use base atomic state helpers

Instead of duplicating the code, make use of the newly introduced atomic
state duplicate and destroy helpers. This allows changes to the base
atomic state handling to automatically propagate to the Tegra driver and
thereby prevent breakage resulting from both copies going out of sync.

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