drm/tegra: Start connectors with correct DPMS mode

A connector's DPMS mode isn't initialized by default, therefore using a
default of 0 (DRM_MODE_DPMS_ON). This can cause problems in that the DRM
core won't explicitly turn on a connector because it thinks that it is
already on.

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