vk/image: Refactor anv_image_create()

vk/image: Refactor anv_image_create()

From my experience with intel_mipmap_tree.c, I learned that for struct's
like anv_image and intel_mipmap_tree, which have sprawling
multi-function construction codepaths, it's easy to mistakenly use
unitialized struct members during construction.

Let's eliminate the risk of using unitialized anv_image members during
construction.  Fill the struct at the function bottom instead of
piecemeal throughout the constructor.