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

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 <lyude@redhat.com>
Fixes: fbc821c4a506 ("drm/mst: Support simultaneous down replies")
Cc: Wayne Lin <Wayne.Lin@amd.com>
Cc: Wayne Lin <waynelin@amd.com>
Cc: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200403200325.885628-1-lyude@redhat.com
Reviewed-by: Sean Paul <sean@poorly.run>