gallium/tests/trivial: fix viewport depth transform

gallium/tests/trivial: fix viewport depth transform

These were getting mapped off into outer space, which would cause nv50
and nvc0 to clip the primitives (as depth_clip was enabled).

These drivers are configured to clip everything outside the [0, 1]
range, even though the hardware supports other view settings.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>