drm/tegra: Disable outputs for power-saving

When an output is disabled, its DPMS mode is usually set to off. Instead
of only disabling the panel (if one is attached), turn the output off
entirely to save more power.

HDMI doesn't have any panels attached, so it previously didn't save any
power at all. With this commit, however, the complete HDMI interface
will be turned off, therefore allowing an attached monitor to go into a
standby mode.

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