drm/amdgpu: drop root shadow sync

Completely pointless, it is the same reservation object as the root PD
anyway.

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