vk_command_pool: Reduce the command pool size from 4096 to 4

This allows drivers to reuse memory more easily and preallocate less.
The optimal number has been measured booting Pokémon Sword.

src/video_core/renderer_vulkan/vk_command_pool.cpp

@@ -10,7 +10,7 @@
 
 namespace Vulkan {
 
-constexpr size_t COMMAND_BUFFER_POOL_SIZE = 0x1000;
+constexpr size_t COMMAND_BUFFER_POOL_SIZE = 4;
 
 struct CommandPool::Pool {
     vk::CommandPool handle;