drm/amdgpu: add synchronization among waves in the same threadgroup

It is possible that the previous waves have exited before others are
created, so the other waves maybe reuse pyhsical resouces left by
previous ones. Therefore add barrier instruction to synchronize waves within
the same threadgroup.

Signed-off-by: Dennis Li <Dennis.Li@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>