drm/amd/display: Add missing pipes registers for VGA enable/disable

Signed-off-by: Nevenko Stupar <Nevenko.Stupar@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>