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.h b/vulkan/libvulkan/driver.h
index 8569a52..7302037 100644
--- a/vulkan/libvulkan/driver.h
+++ b/vulkan/libvulkan/driver.h
@@ -110,6 +110,7 @@
 VKAPI_ATTR VkResult EnumerateInstanceExtensionProperties(const char* pLayerName, uint32_t* pPropertyCount, VkExtensionProperties* pProperties);
 
 VKAPI_ATTR void GetDeviceQueue(VkDevice device, uint32_t queueFamilyIndex, uint32_t queueIndex, VkQueue* pQueue);
+VKAPI_ATTR VkResult AllocateCommandBuffers(VkDevice device, const VkCommandBufferAllocateInfo* pAllocateInfo, VkCommandBuffer* pCommandBuffers);
 // clang-format on
 
 template <typename DispatchableType>