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>
Link: https://patchwork.freedesktop.org/patch/msgid/20230124125726.13323-3-christian.koenig@amd.com