asahi: Use a dynarray for writers

asahi: Use a dynarray for writers

We don't want a writer hash table with persistent pointers to resources, because
the resources could be freed without the hash table being updated (even though
the underlying BO will not be freed until it's ready). To avoid the reference
count hell, do away with the pointer hash table and instead use a flat dynarray
for mapping BO (handles) to writer (batch indices).

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21620>