drm/amd/display: separate plane disable sequence

- disconnect:  trigger hw to disconnect plane on next vsync
- disable: disables HW block and disable clocks
- power_down: kill power

Signed-off-by: Tony Cheng <tony.cheng@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>