ac,radv,radeonsi: add a function to initialize compute preambles
Preambles are very close between RADV and RadeonSI and sharing them makes sense. This MR is only for compute preambles, GFX will be next.
This imports the PM4 mecanishm from RadeonSI to common code which is very useful.
This also initializes GFX12 compute preamble for us.
Edited by Samuel Pitoiset