freedreno: Fix batch reference handling in flush_resource().

freedreno: Fix batch reference handling in flush_resource().

We take references under the lock, but then accessed the lock-requiring
batch_cache structure without holding the lock.  The batches wouldn't get
freed and removed from their slots until the last ref goes away so it was
safe (other than the assert at the end), but writing the simple code is
shorter and requires fewer assumptions.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11439>