drm/amd/display: Work around bad DPCD state on link loss

drm/amd/display: Work around bad DPCD state on link loss

[Why]
This display doesn't properly indicate link loss through DPCD bits such
as CR_DONE / CHANNEL_EQ_DONE / SYMBOL_LOCKED / INTERLANE_ALIGN_DONE,
which all remain set.

In addition, DPCD200Eh doesn't match the value of DPCD204h in all cases.

For these reasons, we can miss re-training the link, since we don't
properly detect link loss with this display.

[Why]
Add display-specific workaround to read DPCD204h, so that we can detect
link loss based on 128b132b-specific status bits in this register.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Ilya Bakoulin <ilya.bakoulin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>