CVE-2026-74381: gpu: host1x: Allow entries in BO caches to be freed

In the Linux kernel, the following vulnerability has been resolved:

gpu: host1x: Allow entries in BO caches to be freed

When a buffer object is pinned via host1xbopin() with a cache, the resulting mapping is kept in the cache so it can be reused on subsequent pins. Each mapping held a reference to the underlying host1xbo (taken in tegrabopin / gatherbopin), so as long as a mapping was cached, the bo itself could not be freed.

However, the only way to remove the cached mapping was through the free path of the buffer object. This meant that if a bo got cached, it could never get freed again.

Resolve the circularity by holding a weak reference to the bo from the cache side. This is done by having the .pin callbacks not bump the bo's refcount -- instead the common Host1x bo code does so, except for the cache reference.

Also move the remove-cache-mapping-on-free code into a common function inside Host1x code. This is only called from the TegraDRM GEM buffers since those are the only ones that can be cached at the moment.