drm/amdgpu: only move pt bos in LRU list on success

This fixes a race condition in the error case: since the pt bos have not
necessarily been reserved in case of an error, we could move a pt bo that
is currently in the middle of being evicted/moved by another process,
which then resulted in a BUG_ON in ttm_bo_add_to_lru.

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