drm/amd/display: fix HW block PG sequence

[why]
Power up and power down has reverted programming order.
also make sure disable root clock last.

Reviewed-by: Muhammad Ahmed <ahmed.ahmed@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Charlene Liu <charlene.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>