Best way to clear a framebuffer image without drawing?
You are looking for vkCmdClearColorImage, 

It does exactly what it says on the tin, it clears an image to a specified color. This command uses the Transfer stage as far as barriers are concerned. You will need to add one between the clear and the vkCmdDispatch.

There is also a vkCmdClearDepthStencilImage if you need it.