drm/i915/guc: Clear pointers on free

Clear out some pointers when objects have been de-allocated. This
makes it much easier to track down use-after-free type issues.

Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201028145826.2949180-4-John.C.Harrison@Intel.com