svga: Flush the hwtnl primitives before writing to buffers.

svga: Flush the hwtnl primitives before writing to buffers.

svga keeps a small queue of similar primitive draws in order to coalesce
them into a single draw primitive command.

But the buffers referred in primitives not yet emitted were being ignored
in the considerations to flush or not the context.

This fixes piglit vbo-map-remap, vbo-subdata-sync, vbo-subdata-zero, and
Seeker.

Based on investigation and patch from Brian Paul.

Reviewed-By: Brian Paul <brianp@vmware.com>