drm/amdgpu: fix VM faults with per VM BOs

There was a small window between unreserve and second reserve where the
freshly allocated BO could have been evicted without the VM noticing it.

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>