media: atomisp: hmm_bo: Further simplify alloc_private_pages()

Further simplify alloc_private_pages().

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>