drm/amdgpu: switch to amdgpu_bo_vm for vm code

The subclass, amdgpu_bo_vm is intended for PT/PD BOs which are also
shadowed, so switch to amdgpu_bo_vm BO for PT/PD BOs.

v4: update amdgpu_vm_update_funcs to accept amdgpu_bo_vm.
v3: simplify code.
    check also if shadow bo exist instead of checking bo only type.
v2: squash three related patches.

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>