drm/i915: Fix a potential UAF at device unload

i915_gem_drain_freed_objects() might not be enough to
free all the objects and RCU delayed work might get
scheduled after the i915 device struct gets freed.

Call i915_gem_drain_workqueue() to catch all RCU delayed work.

Suggested-by: Chris Wilson <chris.p.wilson@intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220923073515.23093-1-nirmoy.das@intel.com