drm/i915: Reorder await_execution before await_request

Reorder the code so that we can reuse the await_execution from a special
case in await_request in the next patch.

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/20200526090753.11329-1-chris@chris-wilson.co.uk