various cffdump improvements for multi-draw indirect

Created by: cwabbott0

The current approach of marking up CP_DRAW_INDIRECT_MULTI isn't going to work with the indirect count variants. The addvariants magic lets us make the opcode a variant we can match on, with no cffdump-specific code. While here, I also added some things that should make it easier to create less verbose packet definitions, that I'll take advantage of in the Mesa MR.