drm: distinguish return value of drm_dp_check_and_send_link_address

drm_dp_check_and_send_link_address discovers MST device topology. It can
return both positive and negative values. When it returns positive
values there is no error found. If it returns negative values there is
error found, such as get NAK , timeout, etc. Following
drm_kms_helper_hotplug_event should be called when
drm_dp_check_and_send_link_address returns positive value.

Signed-off-by: Xiaogang Chen <xiaogang.chen@amd.com>
Acked-by: Simon Ser <contact@emersion.fr>
Signed-off-by: Simon Ser <contact@emersion.fr> (re-formatted commit message)
Link: https://patchwork.freedesktop.org/patch/msgid/DM6PR12MB2602B6281BF8C9430115E03BE3AA0@DM6PR12MB2602.namprd12.prod.outlook.com