media: atomisp: hmm_bo: Rewrite free_private_pages() using pages_array helper funcs

Rewrite free_private_pages() using pages_array helper funcs.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>