drm/amdgpu: add DisplayPort CEC-Tunneling-over-AUX support

Add DisplayPort CEC-Tunneling-over-AUX support to amdgpu.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/de9586bb-151e-bae9-b8e3-14db107a60df@xs4all.nl