drm/ttm: fix pipelined gutting for evictions v2

We can't pipeline that during eviction because the memory needs
to be available immediately.

v2: fix how we cleanup the BOs resources

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Tested-by: Alex Sierra <alex.sierra@amd.com>
Link: https://patchwork.freedesktop.org/patch/379039/