gallium: fix issues in recursive flushing

gallium: fix issues in recursive flushing

When flushing/rendering, some stages (like AA line/point) need to set
pipe/driver state.  Those driver functions often call draw_flush().
That leads to recursion.

Use new draw->suspend_flush flag to explicitly prevent that in the key places.
Remove the draw->vcache_flushing field.
Reuse draw->flushing as a debug/assertion var.