wait_for_timelines is essentially the same loop as retiring requests
(with an extra timeout), so merge the two into one routine.

v2: i915_retire_requests_timeout and keep VT'd w/a as !interruptible
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/20191004134015.13204-10-chris@chris-wilson.co.uk