drm/ttm: properly allocate sys resource during swapout

Drop the special handling here.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210430092508.60710-3-christian.koenig@amd.com