drm/amd/display: add vtg update after global sync update

Global sync update was missing vtg update resulting in underflow if
vstartup decreased a significant amount.

Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Reviewed-by: Jaehyun Chung <Jaehyun.Chung@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>