i965: Share the flush for brw_blorp_miptree_download into a pbo

i965: Share the flush for brw_blorp_miptree_download into a pbo

As all users of brw_blorp_miptree_download() must emit a full pipeline
and cache flush when targetting a user PBO (as that PBO may then be
subsequently bound or *be* bound anywhere and outside of the driver
dirty tracking) move that flush into brw_blorp_miptree_download()
itself.

v2 (Ken): Rebase without userptr stuff so it can land sooner.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>