drm/amd/display: Don't reset clock source at unref

Powering down the clock source during unref is unsafe as we might want
to unref during atomic_check

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>