radeonsi: completely rework updating descriptors without CP DMA

radeonsi: completely rework updating descriptors without CP DMA

The patch has a better explanation. Just a summary here:
- The CPU always uploads a whole descriptor array to previously-unused memory.
- CP DMA isn't used.
- No caches need to be flushed.
- All descriptors are always up-to-date in memory even after a hang, because
  CP DMA doesn't serve as a middle man to update them.

This should bring:
- better hang recovery (descriptors are always up-to-date)
- better GPU performance (no KCACHE and TC flushes)
- worse CPU performance for partial updates (only whole arrays are uploaded)
- less used IB space (no CP_DMA and WRITE_DATA packets)
- simpler code
- hopefully, some of the corruption issues with SI cards will go away.
  If not, we'll know the issue is not here.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>