drm/amd/display: update vgh bounding box

[Why & How]
Update bounding box as per spread sheet.

Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Roman Li <roman.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>