drm/tegra: Don't register DP AUX channels before connectors

As pointed out by the documentation for drm_dp_aux_register(),
drm_dp_aux_init() should be used in situations where the AUX channel for a
display driver can potentially be registered before it's respective DRM
driver. This is the case with Tegra, since the DP aux channel exists as a
platform device instead of being a grandchild of the DRM device.

Since we're about to add a backpointer to a DP AUX channel's respective DRM
device, let's fix this so that we don't potentially allow userspace to use
the AUX channel before we've associated it with it's DRM connector.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210326203807.105754-3-lyude@redhat.com