More merging of mesa_format/pipe_format/dri_image_format

As it says on the box.

These aren't always the same thing - in particular, mesa_format represents GL-visible formats, which are not the same thing as pipe_format/dri_image_format - but we can crush them down to the same thing in a lot of places.

The biggest benefit we get here is that DRI config creation becomes far less mystical; the immediate motivation was adding 4444/1555 modes for Wayland clients, and wishing that I never had to type hardcoded shift masks again.