drm/tegra: Tightly bind RGB output to DC

Previously the association to a DC was done via the encoder's .crtc
field. That has the disadvantage that when an encoder is detached from
its CRTC, that field is set to NULL, leading to situations where it is
impossible to access the DC registers required by the RGB output.

However, the coupling between DC and RGB output is really fixed on
Tegra. While they can be detached logically in DRM, the RGB output can
rely on the DC's existence.

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