drm/i915/gvt: more locking for ppgtt mm LRU list

When the lock was introduced in commit 72aabfb8 ("drm/i915/gvt: Add mutual
lock for ppgtt mm LRU list") one place got lost.

Fixes: 72aabfb8

 ("drm/i915/gvt: Add mutual lock for ppgtt mm LRU list")
Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1580742421-25194-1-git-send-email-igor.druzhinin@citrix.com