vulkan: move AllocateCommandBuffers_Bottom

Move it from loader.cpp to driver.cpp and rename it to
driver::AllocateCommandBuffers.  No functional change.

Change-Id: I0abdca7dea128df0b313b90cfb5d5825566fc790
diff --git a/vulkan/libvulkan/driver_gen.cpp b/vulkan/libvulkan/driver_gen.cpp
index b6081f0..dd203c7 100644
--- a/vulkan/libvulkan/driver_gen.cpp
+++ b/vulkan/libvulkan/driver_gen.cpp
@@ -140,7 +140,7 @@
         "vkAllocateCommandBuffers",
         ProcHook::DEVICE,
         ProcHook::EXTENSION_CORE,
-        reinterpret_cast<PFN_vkVoidFunction>(AllocateCommandBuffers_Bottom),
+        reinterpret_cast<PFN_vkVoidFunction>(AllocateCommandBuffers),
         nullptr,
         nullptr,
     },