drm/amdgpu: stop using amdgpu_bo_gpu_offset in the VM backend

We need to update page tables without any lock held.

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