drm/amd/display: Fix VUpdate offset calculations for dcn401

[WHY&HOW]
DCN401 uses a different structure to store the VStartup offset used to
calculate the VUpdate position, so adjust the calculations to use this
value.

Reviewed-by: Aric Cyr <aric.cyr@amd.com>
Signed-off-by: Dillon Varone <dillon.varone@amd.com>
Signed-off-by: Fangzhi Zuo <jerry.zuo@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>