drm/i915: Make exclusive awaits on i915_active optional

Later use will require asynchronous waits on the active timelines, but
will not utilize an async wait on the exclusive channel. Make the await
on the exclusive fence explicit in the selection flags.

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/20200406155840.1728-1-chris@chris-wilson.co.uk