drm/amd/display: DC I2C review

While reviewing I2C in DC identified a few places. Added a couple to the
TODO list.

1) Connector info read

See get_ext_display_connection_info

On some boards the connector information has to be read through a
special I2C channel. This line is only used for this purpose and only on
driver init.

2) SCDC stuff

This should all be reworked to go through DRM's SCDC code. When this is
done some unnecessary I2C code can be retired as well.

3) Max TMDS clock read

See dal_ddc_service_i2c_query_dp_dual_mode_adaptor

This should happen in DRM as well. I haven't checked if there's
currently functionality in DRM. If not we can propose something.

4) HDMI retimer programming

Some boards have an HDMI retimer that we need to program to pass PHY
compliance.

1 & 3 might be a good exercise if someone is looking for things to do.

v2: Merge dp_dual_mode_adaptor TODO

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>