drm/amd/display: HLK Periodic Frame Notification test failed

[Why]
Due to a small pre-fetch window, the active vline timing is a couple
of lines off when compared to what it should be.

[How]
Changed the calculation for the start vline to account for this window.

Signed-off-by: Murton Liu <murton.liu@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>