drm/amd/display: Revert to old formula in set_vtg_params

[Why]
New formula + cursor change causing underflow
on certain configs

[How]
Rever to old formula

Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Reviewed-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>