asahi: Make bo->writer_syncobj atomic

asahi: Make bo->writer_syncobj atomic

BOs can be written from several contexts, so writing to this member is
racy. We only care about this for the purposes of exporting BOs after a
submission (and if the app is racing writers/submissions at that point
all bets are off), so just keeping track of the last written value is
sufficient.

Switch to atomic operations to eliminate the race, and drop the assert
in the batch cleanup path that no longer holds when the BO might have
been written to from another context.

Fixes: asahi/mesa#20

Signed-off-by: Asahi Lina <lina@asahilina.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22971>