drm/tegra: DPMS off/on in encoder prepare/commit

Previously the panel and output were only enabled on encoder->dpms(). If
userspace called dpms on before doing a modeset, the driver would get into
a state where the connector had a dpms state of ON, but the encoder and output
were not enabled (because the encoder is not yet attached to the connector).
Subsequent dpms ON calls are ignored b/c the connector's state already matches
the desired state.

This patch enables/disables the panel and output on modeset as well, so we
can catch the above case.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>