drm/amd/display: Change offset_to_id to reflect what id_to_offset returns

id_to_offset does not point to the same reg offset that offset_to_id checks for,
causing unintended asserts

Signed-off-by: Murton Liu <murton.liu@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>