drm/amdgpu: fix use after free during BO move

The memory backing old_mem is already freed at that point, move the
check a bit more up.

Signed-off-by: Christian König <christian.koenig@amd.com>
Fixes: bfa3357e ("drm/ttm: allocate resource object instead of embedding it v2")
Bug: drm/amd#1699


Acked-by: Nirmoy Das <nirmoy.das@amd.com>