Remove Mesa format unpack code generation

Now that we have format pack/unpack code up in util/, reuse that from mesa/main for unpack instead of doing our own codegen there (-750 LOC). In the process, fix a couple of little bugs in the util/ version.

i965_dri.so         +204kb
iris_dri.so         -241kb
libvulkan_intel.so  unchanged
libvulkan_freedreno unchanged

That i965 delta is unfortunate. I think it's because util/ has more formats, and because util/'s unpack codegen is on rects instead of rows. I'm thinking about switching it to be rows, since so few things want rects that customizing all the functions for rects is bloating us both on disk and in runtime cost.