anv/image_view: Separate vulkan and isl formats

anv/image_view: Separate vulkan and isl formats

Previously, anv_image_view had a anv_format pointer that we used for
everything.  This commit replaces that pointer with a VkFormat enum copied
from the API and an isl_format.  In order to implement RGB formats, we have
to use a different isl_format for the actual surface state than the obvious
one from the VkFormat.  Separating the two helps us keep things streight.