anv: Use vma_heap for descriptor pool host allocation

anv: Use vma_heap for descriptor pool host allocation

Pre-patch, anv_descriptor_pool used a free list for host allocations
that never merged adjacent free blocks.  If the pool only allocated
fixed-sized blocks, then this would not be a problem. But the pool
allocations are variable-sized, and this caused over half of the pool's
memory to be consumed by unusable free blocks in some workloads, causing
unnecessary memory footprint.

Replacing the free list with util_vma_heap, which does merge adjacent
free blocks, fixes the memory explosion in the target workload.

Disdavantges of util_vma_heap compared to the free list:
  - The heap calls malloc() when a new hole is created.
  - The heap calls free() when a hole disappears or is merged with an
    adjacent hole.
  - The Vulkan spec expects descriptor set creation/destruction to be
    thread-local lockless in the common case. For workloads that
    create/destroy with high frequency, malloc/free may cause overhead.
    Profiling is needed.

Tested with a ChromeOS internal TensorFlow benchmark, provided by
package 'tensorflow', running with its OpenCL backend on clvk.

  cmdline: benchmark_model --graph=mn2.tflite --use_gpu=true --min_secs=60
  gpu: adl
  memory footprint from start of benchmark:
    before: init=132.691MB max=227.684MB
    after:  init=134.988MB max=134.988MB

Reported-by: Romaric Jodin <rjodin@google.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20289>