drm/i915: Clear gt.active_requests before checking idle status

drm/i915: Clear gt.active_requests before checking idle status

commit 3e41a73ec187 ("drm/i915: Suppress busy status for engines if
wedged") moved the check for inflight requests to the
intel_engines_are_idle() check to protect the idle worker. However, the
request selftests were also checking the engine idle status and erroring
out if they did not become idle within a short period of time after the
final wait. In order to accommodate the new check, call retire requests
prior to the engine check so that we flush all the waits.

Fixes: 3e41a73ec187 ("drm/i915: Suppress busy status for engines if wedged")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170331192121.10024-1-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>