anv: Add a helper for doing mass allocations

anv: Add a helper for doing mass allocations

We tend to try to reduce the number of allocation calls the Vulkan
driver uses by doing a single allocation whenever possible for a data
structure.  While this has certain downsides (usually code complexity),
it does mean error handling and cleanup is much easier.  This commit
adds a nice little helper struct for getting rid of some of that
complexity.

Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>