lib/kmstest: Fix up tiled buffer creation

lib/kmstest: Fix up tiled buffer creation

When extracting a raw __gem_set_tiling helper I've fumbled this in

commit 590f6101402b51bca54f69c002380bda967484ea
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed Oct 9 20:50:50 2013 +0200

    lib/drmtest: extract rawer __gem_set_tiling

Fix things up so that we properly propaget the error again.

Now to make this all properly work we also need to make kms_flip a
notch more robust against such failures ...

This only blows up on gen2/3 with the pan tests which want a too wide
framebuffer for tiling.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>