They seem to be broadly similar to the a3xx ones, albeit with some
things shuffled around and with different units, and the extra layout
mode bits.

We also document the FIRST_EXEC_OFFSET registers, so that we can start
properly setting them all to 0 in freedreno and turnip in later commits.
I discovered the compute one when playing with function support in the
blob CL driver, and added the other registers via analogy (the blob
Vulkan driver sets FIRST_EXEC_OFFSET and the shader VA together in one
packet for all stages, so it seems to really be in the same place for
all stages).

Part-of: <mesa/mesa!7386>