freedreno: Fix FD_MESA_DEBUG=flush debug option

fd_batch_check_size() should be after fd_batch_unlock_submit(), otherwise batch_flush() called from fd_batch_check_size() would wait on mutex forever.

Fixes: 02298ed1 "freedreno: Add submit lock"

Found when investigated a crash of tex-miplevel-selection and wasn't able to use that option.