anv: Don't report more memory available than the heap size

When calculating the system memory heap size, we report only 3/4 of
the total RAM size (or 1/2 for systems with less than 4GB of RAM).

In the memory budget extension query, we were reporting 90% of the
available system memory.  If most of the memory in the system is free,
this could result in the total heap size being 3/4 of RAM, but the
memory available being 9/10 of RAM.  But if the application tried to
allocate the memory reported as "available", it would exceed the heap
size.  This can confuse some applications.

This patch makes the memory budget query clamp the available RAM to
the heap size, so it will never report more available than the heap
can provide.  Unfortunately, this means that we'll report only 67.5%
of system memory as available (3/4 * 9/10).  We may want to adjust
this estimate in the future.

Cc: mesa-stable
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26553>

src/intel/vulkan/anv_device.c

@@ -2814,7 +2814,7 @@ anv_get_memory_budget(VkPhysicalDevice physicalDevice,
          }
       } else {
          total_heaps_size = total_sys_heaps_size;
-         mem_available = device->sys.available;
+         mem_available = MIN2(device->sys.available, total_heaps_size);
       }
 
       double heap_proportion = (double) heap_size / total_heaps_size;