drm/amdgpu: make sure to never allocate PDs/PTs for invalidations

Make sure that we never allocate a page table for an invalidation operation.

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>