drm/amd/display: Allow constructing DCE6 link encoder without DDC

drm/amd/display: Allow constructing DCE6 link encoder without DDC

When the DDC channel ID is set to CHANNEL_ID_UNKNOWN,
pass NULL to the AUX regs array.

This is necessary to support embedded connectors without DDC.

Fixes: 7c15fd86aaec ("drm/amd/display: dc/dce: add initial DCE6 support (v10)")
Link: https://gitlab.freedesktop.org/drm/amd/-/work_items/5192
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 38a70e50b22a188ff601740d64dd75f46213121f)