emit HW init commands

@krh @chadversary

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
• use cmd as the variable name of tu_cmd_buffer
• add tu_cs_reserve_space(_assert) for the peace of mind
• 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
• cmdbuf->cs is a pointer to the current CS, so that we can use the helpers above to emit to other CS

I can break it down into smaller commits if desirable.