drm/amdgpu/powerplay/cz: enable/disable vce dpm independent of vce pg

If we don't disable it when vce is not in use, we use extra power
if vce pg is disabled.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>