media: iris: Add buffer to list only after successful allocation

In the Linux kernel, the following vulnerability has been resolved:

media: iris: Add buffer to list only after successful allocation

Move `list_add_tail()` to after `dma_alloc_attrs()` succeeds when creating

internal buffers. Previously, the buffer was enqueued in `buffers->list`

before the DMA allocation. If the allocation failed, the function returned

`-ENOMEM` while leaving a partially initialized buffer in the list, which

could lead to inconsistent state and potential leaks.

By adding the buffer to the list only after `dma_alloc_attrs()` succeeds,

we ensure the list contains only valid, fully initialized buffers.