drm/dp_mst: Don't drop NAKs for down responses

It looks like that when we introduced the ability to handle multiple
down requests at once, we accidentally started dropping NAK replies -
causing sideband messages which got NAK'd to seemingly timeout and cause
all sorts of weirdness.

So, fix this by making sure we don't return from
drm_dp_mst_handle_down_rep() early, but instead treat NAKs like any
other message.

Signed-off-by: Lyude Paul <[e-mail]>
Fixes: fbc821c4a506 ("drm/mst: Support simultaneous down replies")
Cc: Wayne Lin <[e-mail]>
Cc: Wayne Lin <[e-mail]>
Cc: Sean Paul <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
Reviewed-by: Sean Paul <[e-mail]>

drivers/gpu/drm/drm_dp_mst_topology.c

···
 			      txmsg->reply.u.nak.reason,
 			      drm_dp_mst_nak_reason_str(txmsg->reply.u.nak.reason),
 			      txmsg->reply.u.nak.nak_data);
-		goto out_clear_reply;
 	}
 
 	memset(msg, 0, sizeof(struct drm_dp_sideband_msg_rx));