staging/vboxvideo: Use gem_free_object_unlocked

vboxvideo doesn't use dev->struct_mutex and therefore has no need to use
gem_free_object.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Michael Thayer <michael.thayer@oracle.com>
Cc: Colin Ian King <colin.king@canonical.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180327082356.24516-1-daniel.vetter@ffwll.ch