drm/i915: Extend i915_request_await_active to use all timelines

Extend i915_request_await_active() to be able to asynchronously wait on
all the tracked timelines simultaneously.

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/20200311092044.16353-1-chris@chris-wilson.co.uk