Stop checking the mapped BO itself, cause that one is
certainly not a page table.

Additional to that move the code into amdgpu_vm.c
Signed-off-by: NChristian König <christian.koenig@amd.com>
Reviewed-by: NChunming Zhou <david1.zhou@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>