drm/amd/display: Roll back unit correction

[why]
This Unit correction exposes a Replay corruption.

[how]
This reverts commit:
commit dbd29029 ("drm/amd/display: Correct unit conversion for vstartup")

Roll back unit conversion until Replay can fix their corruption.

Fixes: dbd29029 ("drm/amd/display: Correct unit conversion for vstartup")
Reviewed-by: Reza Amini <reza.amini@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Ovidiu Bunea <ovidiu.bunea@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>