This could resolve HW deadlocks where a unit downstream of the VS is
waiting for more input, the VS has one vertex queued up but not
dispatched because it hopes to get one more vertex for 2x4 dispatch,
and software isn't handing more vertices down because it's waiting for
rendering to complete.  The B-Spec says you should always have this
bit set.
Signed-off-by: NEric Anholt <eric@anholt.net>