drm/i915/selftests: Replace wait-on-timeout with explicit timeout

In igt_flush_test() we install a background timer in order to ensure
that the wait completes within a certain time. We can now tell the wait
that it has to complete within a timeout, and so no longer need the
background timer.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180709122044.7028-3-chris@chris-wilson.co.uk