drm/amdgpu: make sure BOs are locked in amdgpu_vm_get_memory

We need to grab the lock of the BO or otherwise can run into a crash
when we try to inspect the current location.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Guchun Chen <guchun.chen@amd.com>
Tested-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>