If you have an out-of-tree build, gen8_pack.h and friends will not be in
the same folder as genX_pack.h so this will be a problem. We fixed
out-of-tree earlier by adding the genxml folder to the includes for the
vulkan driver. However, this is not a good long-term solution because we
want to use it in ISL as well.
I've had people ask about the design of the pack functions, for example,
why aren't we using bitfields. I wrote up a bit of background on why and
how we ended up with the current design and we might as well keep that
with the code.
