drm/amd/display: Disable dpp root clock when not being used

[Why & How]
Disable root clock for dpp when not being used.

Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Agustin Gutierrez Sanchez <agustin.gutierrez@amd.com>
Signed-off-by: Jake Wang <haonan.wang2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>