panfrost: Move BO cache related fields to a sub-struct

panfrost: Move BO cache related fields to a sub-struct

We will soon introduce an LRU list to evict BOs that have been unused
for more than 1 second. Let's first move all BO cache fields to a
sub-struct to clarify which fields are used by the BO caching logic.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>