anv: Allow storage on all formats that support typed writes

In particular, this gives us B8G8R8A8_UNORM storage support which is useful for writing WSI images from compute shaders. These formats can only be accessed in a spec-compliant way by decorating the variable NonReadable in the SPIR-V (writeonly in GLSL). If the client doesn't so decorate the variable, it'll get the null surface state where reads return 0 and writes are ignored.