drm/tegra: dc: Link DC1 to DC0 on Tegra20

Hardware reset isn't actually broken on Tegra20, but there is a
dependency on the first display controller to be taken out of reset for
the second to be enabled successfully. Model this dependency using a PM
device link.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
[treding@nvidia.com: minor cleanups, extend commit message]
Signed-off-by: Thierry Reding <treding@nvidia.com>