drm/amdgpu: rework shadow handling during PD clear v3

This way we only deal with the real BO in here.

v2: use a do { ... } while loop instead
v3: fix NULL pointer in v2

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>