gallium/u_threaded: flush batch when hitting mapping limit

gallium/u_threaded: flush batch when hitting mapping limit

tc_transfer_map maps buffers directly, but the unmap operation is executed
in the driver thread.
When an application does a lot of map/unmap operations, without flushing,
this increase the RAM used (and eventually get the app killed by the oom-killer).

This commit allows tc to keep track of how many bytes were mapped during
the current batch. When this estimation becomes higher than a threshold,
we flush the batch.

See: https://gitlab.freedesktop.org/mesa/mesa/-/issues/2735
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4508>