drm/amdgpu: add pipeline sync while vmid switch in same ctx

Since vmid-mgr supports vmid sharing in one vm, the same ctx could
get different vmids for two emits without vm flush, vm_flush could
be done in another ring.

Signed-off-by: Chunming Zhou <David1.Zhou@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>