drm/ttm: revert "drm/ttm: make TT creation purely optional v3"

This reverts commit 2ddef176.

As it turned out VMWGFX needs a much wider audit to fix this.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200811092400.188124-1-christian.koenig@amd.com