intel: Add a batch flush between front-buffer downsample and X protocol.

intel: Add a batch flush between front-buffer downsample and X protocol.

This was already happening because blorp happens to flush at the end of
every call, but we have been talking about removing that at some point,
and this would surely get overlooked.

v2 (Kenneth Graunke): Rebase on latest master.  Note that we did remove
   the other flush, and this change actually did get overlooked!

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>