accel/habanalabs: always pass exported size to alloc_sgt_from_device_pages()

For Gaudi1 the exported dma-buf is always composed of a single page, and
therefore the exported size is equal to this page's size.
When calling alloc_sgt_from_device_pages(), we pass 0 as the exported
size and internally calculate it as "number of pages * page size".
This makes alloc_sgt_from_device_pages() less clear, because the
exported size parameter is not understood as a restriction on the pages'
size.
Modify to always pass the exported size explicitly.

Signed-off-by: Tomer Tayar <ttayar@habana.ai>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>