drm/amd/display: Change swizzle visual confirm reference pipe

[Why]
To change the swizzle visual confirm reference pipe from top pipe to
bottom pipe due to bottom pipe information would be more important
for multiple overlay case.

Signed-off-by: Po-Ting Chen <robin.chen@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>