i965: Move has_hiz from the slice to the level.

i965: Move has_hiz from the slice to the level.

The value depends only on the level, so no need to store the bool per slice.
Shrinks intel_mipmap_slice from 24 bytes to 16, while slotting into an
existing hole in intel_mipmap_level.

Reviewed-by: Chad Versace <chad.versace@linux.intel.com>