vk/formats: Document new meaning of anv_format::cpp

The way the code currently works is that anv_format::cpp is the cpp of
anv_format::surface_format.

Me and Kristian disagree about how the code *should* work. Despite that,
I think it's in our discussion's best interest to document how the code
*currently* works. That should eliminate confusion.

If and when the code begins to work differently, then we'll update the
anv_format comments.

src/vulkan/private.h

@@ -775,7 +775,7 @@ void anv_compiler_free(struct anv_pipeline *pipeline);
 struct anv_format {
    const char *name;
    uint16_t surface_format; /**< RENDER_SURFACE_STATE.SurfaceFormat */
-   uint8_t cpp;
+   uint8_t cpp; /**< Bytes-per-pixel of anv_format::surface_format. */
    uint8_t num_channels;
    uint8_t depth_format; /**< 3DSTATE_DEPTH_BUFFER.SurfaceFormat */
    bool has_stencil;