drm/amdgpu: Reserve fences for VM update

In amdgpu_dma_buf_move_notify reserve fences for the page table updates
in amdgpu_vm_clear_freed and amdgpu_vm_handle_moved. This fixes a BUG_ON
in dma_resv_add_fence when using SDMA for page table updates.

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