drm/amd/display: Allow DCE link encoder without AUX registers

drm/amd/display: Allow DCE link encoder without AUX registers

Allow constructing the DCE link encoder without DDC,
which means the AUX registers array will be NULL.

This is necessary to support embedded connectors without DDC.

Fixes: 4562236b3bc0 ("drm/amd/dc: Add dc display driver (v2)")
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 87f30b101af62590faf6020d106da07efdda199b)