media: atomisp: hmm_bo: Simplify alloc_private_pages()

Since lack_mem starts initialized to true, alloc_private_pages() will
always set order to HMM_MIN_ORDER aka 0 / will always alloc 1 page at
a time.

So all the magic to decrease order if allocs fail is not necessary
and can be removed.

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>