anv: Rework anv_block_pool_expand_range

anv: Rework anv_block_pool_expand_range

The growing algorithms for the softpin case and the userptr version are
almost entirely different.  Having this weird join doesn't make the code
more comprehensible.  This rework does a few things:

 1. Move the comment about 48-bit addresses to anv_device_init where we
    actually unset the EXEC_OBJECT_SUPPORTS_48B_ADDRESS flag.

 2. Separate the paths in anv_block_pool_expand_range so it's easier to
    see what happens in the two different cases.

 3. Use the anv_block_poo::bos array for storing all allocated BOs in
    both paths rather than using the cleanup list in both paths.  This
    lets us make the cleanups array only used for mmaps of the memfd for
    the userptr case.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>