mesa/st: add software-fallback for RGTC

Here's the second to last piece of the puzzle for BC1-7 emulation in the mesa state-tracker, in line with !18180 (merged) and !18012 (merged).

The LATC bits has been factored out into !18564 (merged), because LATC seems to need more work. Unfortunately, this currently leaves us still with the RGTC hacks in u_transfer_helper. Freedreno and Panfrost currently uses those for LATC support as well as RGTC, although from reading the code, it doesn't really look like the LATC bits work (they seem to do the exact same thing as for RGTC, which is only correct for upload/download, not for rendering).

Tested here. The iris-traces is just lowest-bit changes in decoding precision, which is allowed (and pretty much expected), and the Lavapipe failure is due to me hacking out a required format.