drm/tegra: dsi - Implement VDD supply support

The DSI controllers are powered by a (typically 1.2V) regulator. Usually
this is always on, so there was no need to support enabling or disabling
it thus far. But in order not to consume any power when DSI is inactive,
give the driver a chance to enable or disable the supply as needed.

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