drm/amdgpu: fix VM handling for GART allocations

For GTT allocations with a GART address the res contains the VMID0
addresses and can't be used for VM handling.

So ignore the res when the pages array is given or we fill the page
tables with nonsense.

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