drm/tegra: rgb: Implement ->atomic_check()

drm/tegra: rgb: Implement ->atomic_check()

The implementation of the ->atomic_check() callback precomputes all
parameters to check if the given configuration can be applied. If so the
precomputed values are stored in the atomic state object for the encoder
and applied during modeset. In that way the modeset no longer needs to
perform any checking but simply program values into registers.

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