i915: fix BadAlloc at CreateContext for i830

There is missed initialization of success variable when i830 detected. i830CreateContext has never been called.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=56042
Reported-by: goetzchrist@gmail.com
Signed-off-by: Matwey V. Kornilov <matwey.kornilov@gmail.com>