freedreno: fix traces timeouts

Somehow fairly recently the traces CI job started hitting timeouts, not all the time but enough to be inconvenient for CI. I tracked it down to getting into a situation where ctx->batch->flush == true, which causes an infinite loop in the draw_vbo and clear paths (because fd_batch_lock_submit() checks for flushed batch but fd_context_batch() does not). I'm not entirely sure how we get into that state, or what triggered this (seems possibly triggered by !10937 (merged)). But it is easy enough to recover.