glamor: sync_fence_set_triggered should use glFlush, not glFinish

I intended to use glFlush all along, but somehow managed to type
glFinish instead. glFlush is sufficient (for a single-queue GPU) to
ensure serialization between queued rendering in the X server and
future rendering from the client.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Jasper St. Pierre <jstpierre@mecheye.net>