drm/i915: Fix vma resource freeing

In some cases we use leftover kfree() instead of i915_vma_resource_free().
Fix this.

Fixes: 2f6b90da ("drm/i915: Use vma resources for async unbinding")
Reported-by: Robert Beckett <bob.beckett@collabora.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220119174734.213552-1-thomas.hellstrom@linux.intel.com