anv: Support B8G8R8A8_UNORM storage images

Even though B8G8R8A8_UNORM isn't one of the classic storage image formats, it is the usual swapchain image format on Linux so it's helpful to support it for storage. Because SPIR-V provides no enums for BGRA formats, it can only be used without a format which, on ANV, means it must be a write-only image. However, since the intended use-case is for writing to a swapchain image from a compute shader, this doesn't seem onerous.