Use adev->vm_manager.id_mgr[0].num_ids rather than hardcoded 16.

v2: use AMDGPU_GFXHUB rather than hardcoded 0 (Christian)
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Noticed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>