drm/amd/display: fix seamless boot stream adding algorithm

[Why]
Seamless boot stream has hw resource assigned, already.  'add' is
actually rebuild the assignment.

[How]
Swap seamless boot stream to pipe 0 (if needed) to ensure pipe_ctx
matches

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Raymond Yang <rayyang@amd.com>
Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>