drm/ttm: stop allocating a dummy resource for pipelined gutting

That should not be necessary any more when drivers should at least be
able to handle a move without a resource.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Acked-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230130120636.63765-5-matthew.auld@intel.com