drm/amdgpu: Fix compute VM BO params after rebase v2

The intent of two commits was lost in the last rebase:

810955ba drm/amdgpu: Fix acquiring VM on large-BAR systems
b5d21aac drm/amdgpu: Don't use shadow BO for compute context

This commit restores the original behaviour:
* Don't set AMDGPU_GEM_CREATE_NO_CPU_ACCESS for page directories
  to allow them to be reused for compute VMs
* Don't create shadow BOs for page tables in compute VMs

v2: move more logic into amdgpu_vm_bo_param

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