drm/amdgpu: Fix acquiring VM on large-BAR systems

On large-BAR systems the VM page tables for compute are accessed by
the CPU. Always allow CPU access to the page directory so that it can
be used later by the CPU when a VM is converted to a compute VM.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>