drm/i915: Wait for engines to idle before retiring

drm/i915: Wait for engines to idle before retiring

In the next^W forthcoming patch, we will start to defer retiring the
request from the engine list if it is still active on the submission
backend. To preserve the semantics that after wait-for-idle completes
the system is idle and fully retired, we need to therefore wait for the
backends to idle before calling i915_retire_requests().

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/20180627115334.16282-1-chris@chris-wilson.co.uk