anv/image: Add a vk_format field

anv/image: Add a vk_format field

We've been trying to move away from anv_format for a while and this should
help with the transition.  There are cases (mostly in meta) where we need
the original format for the image and not the isl_format.  These will be
moved over to the new vk_format and everythign else will use the isl_format
from the particular anv_surface.