drm/amd/display: Poll pending down rep before clear payload allocation table

[Why]
On current design, driver cannot handle the interrupt for
down reply when link training is processing. The DOWN REQ
send before link training will keep in the pending DOWN REP
state in the queue.
It makes the next DOWN REQ be queued until time out.

[How]
To add a polling sequence before clear payload allocation table
to make sure the pending DOWN REP can be handled.

Signed-off-by: Martin Tsai <martin.tsai@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>