freedreno: rework fence tracking

freedreno: rework fence tracking

ctx->last_fence isn't such a terribly clever idea, if batches can be
flushed out of order.  Instead, each batch now holds a fence, which is
created before the batch is flushed (useful for next patch), that later
gets populated after the batch is actually flushed.

Signed-off-by: Rob Clark <robdclark@gmail.com>