drm/amdgpu: link all gtt when binding them V2

V2:
spin lock instead of mutex for gtt list

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