iris: make the TFB result visible to others

iris: make the TFB result visible to others

OpenGL 4.6 Spec:
   "5.3.3 Rules
    .......
    Note: “Updates” via rendering or transform feedback
    are treated consistently with updates via GL commands.
    Once EndTransformFeedback has been issued, any subsequent
    command in the same context that uses the results of the
    transform feedback operation will see the results."

v2: removed a wrong comment
    ( Kenneth Graunke <kenneth@whitecape.org> )

v3: - flush+dirty depends on buffers usage history
    - removed an old hack
    ( Kenneth Graunke <kenneth@whitecape.org> )

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110404
Signed-off-by: Andrii Simiklit <andrii.simiklit@globallogic.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>