nv50,nvc0: don't base decisions on available pushbuf space

nv50,nvc0: don't base decisions on available pushbuf space

We still have to push everything out, might as well kick earlier and
flip pushbufs when we know we'll need it. This resolves some issues with
the new policy of making sure that we always leave a bit of room at the
end for fences.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Fixes: 47d11990b (nouveau: make sure there's always room to emit a fence)
Cc: mesa-stable@lists.freedesktop.org