drm/amd/display: Get link index for AUX reply notification

The link index wasn't updated for the AUX reply notification. Get link
index based on DPIA instance for AUX reply notification.

Signed-off-by: Cruise <Cruise.Hung@amd.com>
Reviewed-by: Meenakshikumar Somasundaram <meenakshikumar.somasundaram@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>