drm/amdgpu: fix amdgpu_vm_bo_wait

We need to wait with the correct owner on unmap operations or otherwise can run
into VM faults.

Also always wait for the page directory since this is where the reservation
object comes from. So rename the function to amdgpu_vm_wait_pd instead as well.

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