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

This reverts commit 4110872b.

This still seems to break i915.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Matthew Auld <matthew.william.auld@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230125155023.105584-1-christian.koenig@amd.com