drm/tegra: rgb: 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
code to use runtime PM in subsequent patches.

While at it, remove the enabled field that hasn't been used since the
demidlayering of the output drivers done in preparation for the atomic
mode-setting conversion.

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