media: atomisp: Log an error on failing to alloc private-mem

I managed to trigger an atomisp_css_start() error by pushing my test
system towards an OOM situation, this resulted in the following errors:

atomisp-isp2 0000:00:03.0: alloc pages err...
atomisp-isp2 0000:00:03.0: hmm_bo_alloc_pages failed.
atomisp-isp2 0000:00:03.0: stream[0] start error.

But it is not entirely clear what the root cause of
the "alloc pages err..." error is. I suspect the root cause is
alloc_pages_bulk_array() failing. Add a log message to make
the root cause more clear if this is hit again.
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

drivers/staging/media/atomisp/pci/hmm/hmm_bo.c

@@ -638,6 +638,7 @@ static int alloc_private_pages(struct hmm_buffer_object *bo)
 	ret = alloc_pages_bulk_array(gfp, bo->pgnr, bo->pages);
 	if (ret != bo->pgnr) {
 		free_pages_bulk_array(ret, bo->pages);
+		dev_err(atomisp_dev, "alloc_pages_bulk_array() failed\n");
 		return -ENOMEM;
 	}