turnip: emit HW init in tu_BeginCommandBuffer

turnip: emit HW init in tu_BeginCommandBuffer

Being the first commit that emits meaningful command packets, there
are many things included in this commit

 - tu6_emit_xxx are low-level helpers that emit command packets
   without boundary checks
 - tu6_xxx are high-level helpers that emit command packets with
   boundary checks
 - cmdbuf->cs is a pointer to the current CS, so that we can use the
   helpers above to emit to other CS
 - use cmd as the variable name of tu_cmd_buffer
 - there is a per-cmdbuf scratch bo for CP_EVENT_WRITE writeback
 - there is a per-cmdbuf debug marker, using scratch reg 7 or 6
   depending on whether the cmdbuf is primary or secondary

(olv, after rebase) REG_A6XX_SP_UNKNOWN_AB20 is renamed