i965: don't require 64bit cmpxchg

i965: don't require 64bit cmpxchg

There are still some distributions trying to support unfortunate people
with old or exotic CPUs that don't have 64bit atomic operations. The
only thing preventing compile of the Intel driver for them seems to be
initialization of a debug variable.

v2: use call_once() instead of unsafe code, as suggested by Matt Turner

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93089
Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>