drm/amdgpu: add shadow flag V2

Indicate if need to sync between bo and shadow, where sync to where.
V2:
Rename to backup_shadow

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>