drm/amdgpu: don't update page tables for VM emulation

It's just overhead to do so and allocating a VMID
when we don't need one is actually a bit dangerous.

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