drm/amd/display: use updated clock source init routine

[why]
Use correct clock source initialization routine for DCN32/321

Signed-off-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>