Make sure we can at least move and alloc TT objects without backing store.

v2: clear the tt object even when no resource is allocated.
v3: add Matthews changes for i915 as well.
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230124125726.13323-1-christian.koenig@amd.com