drm/i915/gt: Flush the requests after wedging on suspend

Retire all requests if we resort to wedged the driver on suspend. They
will now be idle, so we might as we free them before shutting down.

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/20191118230254.2615942-16-chris@chris-wilson.co.uk