iris: Add BO_ALLOC_PLAIN flag

iris: Add BO_ALLOC_PLAIN flag

We had iris_bo_alloc() call with the wrong flags value, it was
mistaken by aligment, see commit 68652dca0c28.

To avoid such mistakes in future here adding BO_ALLOC_PLAIN so
iris_bo_alloc() calls have a more descriptive flag parameter when
there is no special allocation request.

While at it, also standardizing unsigned as the type for this flag.

No behavior changes here.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19857>