v3dv: reorganize code to support several hw versions

v3dv was created initially with the rpi4 in mind, so hw version == 42. In order to advance without worrying on less things as possible, initially we didn't care too much about supporting more that one generation, so in that aspect we can find code that depends on the hw version (like the packet/emit packet methods) on any source file.

Now that the source code is more mature, it would be interesting to clean up everything in order to support more that one hw version, past or future. The poster boy here would be hw version 41 (if we got any device with that Broadcom version), that even if it is really similar to 42, there are still some differences.