drm/amd/display: Use pwrseq instance to determine eDP instance

[Why & How]
Link index doesn't always correspond to the appropriate eDP instance.
We can assume lower link index is a lower eDP instance and set panel
control instance accordingly.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Jake Wang <haonan.wang2@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>