anv: Replace ANV_BO_EXTERNAL with anv_bo::is_external

anv: Replace ANV_BO_EXTERNAL with anv_bo::is_external

We're not THAT strapped for space that we can't burn one extra bit for
a boolean.  If we're really worried about it, we can always shrink the
flags field to 16 bits because the kernel only uses 7 currently.

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