drm/tegra: dsi: Restore DPMS

In order to restore DPMS with atomic mode-setting, move all code from
the ->mode_set() callback into ->enable(). At the same time, rename the
->prepare() callback to ->disable() to use the names preferred by atomic
mode-setting. This simplifies the calling sequence and will allow DPMS
to use runtime PM in subsequent patches.

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