drm/amd/display: fix init_adj offset for cositing in SPL

[Why & How]
init_adj offset is applied when cosited not interstitial
Adjust cositing offset in SPL

Reviewed-by: Jun Lei <jun.lei@amd.com>
Signed-off-by: Samson Tam <Samson.Tam@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>