drm/i915: Move __i915_gem_free_object to ttm_bo_destroy

When we implement delayed destroy, we may have a second
call to the delete_mem_notify() handler, while free_object()
only should be called once.

Move it to bo->destroy(), to ensure it's only called once.
This fixes some weird memory corruption issues with delayed
destroy when async eviction is used.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210830121006.2978297-2-maarten.lankhorst@linux.intel.com
Fixes: 213d5092

 ("drm/i915/ttm: Introduce a TTM i915 gem object backend")
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>