kmstest: don't pollute stderr for impossible output configs

kmstest: don't pollute stderr for impossible output configs

Otherwise on machines where the only output available is restricted to
one pipe we'll have tons of "warn" results for no gain in the piglit
runner. All tests that use the kmstest helpers already check the
return value and do something sensible (like skip the tests if there's
really no config available).

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