drm/amdgpu: separate gpu address from bo pin

It could be got by amdgpu_bo_gpu_offset() if need

Signed-off-by: Junwei Zhang <Jerry.Zhang@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>