* dnn/Vulkan: don't init Vulkan runtime if using other backend/target

Don't need to explictly call a init API but will automatically
init Vulkan environment the first time to use an VkCom object.
Signed-off-by: Wu Zhiwen <zhiwen.wu@intel.com>

* dnn/Vulkan: depress compilier warning for "-Wsign-promo"
Signed-off-by: Wu Zhiwen <zhiwen.wu@intel.com>