drm/i915/selftests: Disable preemption while setting up fence-timers

drm/i915/selftests: Disable preemption while setting up fence-timers

The impossible happens and a future fence expired while we were still
initialising. The probable cause is that the test was preempted and we
lost our scheduler cpu slice. Disable preemption during this test to
rule out preemption as a source of timer disruption.

References: https://bugs.freedesktop.org/show_bug.cgi?id=110039
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190313205944.5768-1-chris@chris-wilson.co.uk