drm/amdgpu: allocate shadow for pd/pt bo V2

The pd/pt shadow bo will be used to backup page table, when gpu reset
happens, we can restore the page table by them.
V2:
Free shadow bo.

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>