anv: Update usage of block_pool->bo.

anv: Update usage of block_pool->bo.

Change block_pool->bo to be a pointer, and update its usage everywhere.
This makes it simpler to switch it later to a list of BOs.

v3:
 - Use a static "bos" field in the struct, instead of malloc'ing it.
 This will be later changed to a fixed length array of BOs.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>