drm/panfrost: Handle error when allocating AS number

drm/panfrost: Handle error when allocating AS number

If we reach the beginning of the LRU AS list, then return an error.

Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Link: https://lore.kernel.org/r/20251019145225.3621989-5-adrian.larumbe@collabora.com
Signed-off-by: Steven Price <steven.price@arm.com>