drm/i915: Flush the residual parking on emergency shutdown

On unwinding following a critical failure inside GEM init, we also need
to be sure to flush the workers before unloading the module.

Testcase: igt/drv_module_reload/basic-reload-inject
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180710094421.16223-1-chris@chris-wilson.co.uk