drm/amdgpu: Redo XGMI reset synchronization.

Use task barrier in XGMI hive to synchronize ASIC resets
across devices in XGMI hive.

v2: Return right away with a warning if no xgmi hive, update doc.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Le Ma <Le.Ma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>