QVulkanInstance: Fix compilation with MSVC2013

Use assignment. Fixes:

vulkan\qvulkaninstance.cpp(811) : error C2061: syntax error : identifier 'd_ptr'
vulkan\qvulkaninstance.cpp(812) : warning C4551: function call missing argument list
vulkan\qvulkaninstance.cpp(813) : error C2659: '=' : function as left operand
vulkan\qvulkaninstance.cpp(814) : error C2440: 'return' : cannot convert from 'QVulkanDeviceFunctions *&(__cdecl *)(void)' to 'QVulkanDeviceFunctions *'
        There is no context in which this conversion is possible
vulkan\qvulkaninstance.cpp(832) : error C2061: syntax error : identifier 'd_ptr'
vulkan\qvulkaninstance.cpp(833) : error C2541: 'delete' : cannot delete objects that are not pointers
vulkan\qvulkaninstance.cpp(834) : error C2659: '=' : function as left operand

Change-Id: I859b141aa0cb24b1f85dc9f229262a4145651d7c
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>