drm/amd/display: Poll pending DOWN_REP before enabling the link

[Why]
With special monitor combination on MST, the UP_REQ could come
after clear payload table. It makes the pending DOWN_REP fail
to be handled after link training and the new DOWN_REQ will be queued
until time out .

[How]
To move the current polling pending DOWN_REP procedure to after
clear payload table to make sure the pending DOWN_REP can be
done before enabling the link.

Signed-off-by: Martin Tsai <martin.tsai@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>