drm/amdgpu: cleanup amdgpu_sync_rings V2

No longer needed now that semaphores are gone.

V2: remove the first amdgpu_sync_wait in amdgpu_ib_schedule

Signed-off-by: Chunming Zhou <David1.Zhou@amd.com>
Reviewed-by: Ken Wang  <Qingqing.Wang@amd.com> (V1)
Reviewed-by: Monk Liu <monk.liu@amd.com> (V2)
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>