drm/tegra: dc: Add Tegra186 support

The display architecture has changed in several signifcant ways with the
new Tegra186 SoC. Display controllers are a completely different design,
but have been given a frontend that simulates the register interface for
earlier chips.

Unfortunately the frontend isn't completely backwards compatible, so the
driver needs parameterization to take the changes into account.

One big change is that the total number of display controllers has been
increased to three. At the same time the number of planes available has
remained constant. However, planes can now be freely assigned between
the display controllers, giving applications more flexibility in making
the best use of the available resources.

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