drm/amdgpu: avoid the modulo in amdgpu_vm_get_entry

We can do this with a simple mask as well.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>