drm/amd/display: Implement DDC probe over AUX channel

drm/amd/display: Implement DDC probe over AUX channel

DDC probe means that we use the DDC (Display Data Channel) to
verify whether a display is connected or not.

This was written with mainly analog connectors in mind, so it
didn't work over the AUX channel. However, in order to support
external DP bridge encoders found in CIK APUs, it needs to work
over the AUX channel too.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>