drm/amdgpu: revert "drm/amdgpu: stop allocating dummy GTT nodes"

TTM is going to need this again since we are moving the resource
allocation into the backend.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210602100914.46246-4-christian.koenig@amd.com