drm/amd/display: Update viewport position for phantom pipes

[Description]
In some cases the viewport position of the main pipes can change without
triggering a full update. In this case the subvp phantom viewports must
be updated accordingly.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Pavle Kotarac <Pavle.Kotarac@amd.com>
Signed-off-by: Alvin Lee <Alvin.Lee2@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>